I need a laser that will start a fire.

Back when I tried it as a curious teenager all I had was a good headlight from a mid 70's ford car or pickup, you know those old round ones, and a big magnifying glass that has been in the family far longer than I have been. I didn't have any trouble lighting paper and bits of wood and grass fire with it or making plastics smoke with it.

I will have to dig around and see if anyone in the family still has the old magnifying glass and the odds are my 3 million candle power spotlight should work just fine being it uses a pair of 100w halogen bulbs.

But no I still don't think it will light a tree on fire from 50+ feet away.

Not looking good. Still - let's say Gary was to get a big 2kW spotlight and really huge fresnel lens...

duffy said:

Not looking good. Still - let's say Gary was to get a big 2kW spotlight and really huge fresnel lens...

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I'm not sure the size of the lens really matters as much as you seem to think it would. It's the power and position of the light source. The only reason a magnifying glass works with the sun is because the sun is so far away and the rays that hit the lens are pretty much perpendicular to its surface. It is a diagram like this and some simple mathematics/physics that you'll need to use in order to calculate focal length. I'll take a look and see if I can find the math somewhere.

**broken link removed**

Regards,
Der Strom

EDIT: I just did a quick google search and I found the following site that gives a great explanation:

**broken link removed**

The further the source is from the glass, the more focused the light becomes, according to the formulas presented. I hope this helps!

Hi DerStrom, A larger lense focuses more light. It's that simple.

BrownOut said:

Hi DerStrom, A larger lense focuses more light. It's that simple.

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Hi BrownOut. I can understand that, but when the only light you have is from a headlight, you really don't need a huge lens. The lamp only puts out so much light, so increasing the size of the lens isn't going to help very much. But, of course, that depends on the reflector you have behind the headlight. If it reflects it outward, then yes, a larger lens would be necessary. However, I figured tcmtech intended the reflector to point the light forward rather than out....

However, I figured tcmtech intended the reflector to point the light forward rather than out....

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I would suspect that since there has been a considerable design change between the old round headlamps from decades ago VS the newer designs of modern vehicles that is why Duffys experiment didn't work so well with a Fresnel lens. The new ones spread out their beam patterns considerably wider than the old ones did.

I think most of the old headlights had a more oval or roundish beam pattern than the highly complex ones that they have now which is probably why my experiment worked well enough to get small fires to start. Just a guess though.

Now I am tempted to take on of my 400 watt Metal Halide bulbs and put that in a metal coffee can with a reflective foil lining and put a lens in front of that and see what sort of mischief I can come up with!

I can't believe how this thread has warped from felling a large tree..... to the history of vehicle headlamps.....

Hahaha, yep. Quite a bit of a diversion. I suppose we should get back on topic, huh?

I can't believe how this thread has warped from felling a large tree..... to the history of vehicle headlamps.....

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Seems relevant to me! Its just part of the eternal quest to make fire in some unnecessarily complicated fashion that deems you to be far cooler than your match or lighter toting buddies!

Well I just noticed it was moved to the members lounge... maybe we should let it evolve into something, something beautiful *laughs maniacally with lightening in the background.....whilst holding a fresnel lens..*

I wonder... would you want to use a metal halide, or a big incandescent? To set that dead tree ablaze, seems like you would want infrared. Metal halides and arc lamps don't put out as much of that, proportionally, as an incandescent. IR can be focused with a lens, same as visible light.

Maybe the thing to use would actually be a big heating element. Or, maybe you would use a gas-fired device that heats a small ceramic object at the focus point of a BIG reflector, with a BIG fresnel lens in front of it, until it gets white hot. Something like that could conceivably put out tens of kilowatts of infrared.