Found a cool mirror while surfing today. Looks like it could do double douty as a fire starter and signaling device. I would stick with fire though because since I havent played with it I have no idea on aiming it (also if you hit a pilot with it, could it possibly be TOO BRIGHT?). Only problem I found with it would be size. At 4.5 inches in diameter it's already leaving the EDC range. For a pack or BOB though it would be handy. You can Froogle "parabolic mirror solar light" or just go to This Random Site for an example if you don't want to do the search.

A parabolic mirror has a focal length - the point where it focuses the reflection into that small bright spot. Typically measured in inches (or feet for gigantic ones). If you manage to blind a flying pilot with this intense spot, you're way too close to the plane!

Good to know. So after the "focal point" does it then scatter too much that it would be worthless for signaling I would take it?

Yes, the light will "spread out" past the focal point. I don't know that it would make it "worthless" for signalling if that's all you had available, but a plain flat signalling mirror will be better, lighter, smaller, easier to aim, and cheaper. Plus parabolic mirrors are "front surface" mirrors. Which means the reflective coating is on the front of the glass, not on the back side of the glass like a normal mirror. Front surface gives you better reflectivity and better imaging, but it is terribly prone to scratching and deterioration of the reflective coating. You don't want to go touching it with your fingers. You have to really protect it. For telescopes you want a front surface mirror because if the reflective coating is on the back of the glass like a normal mirror, you will also have a faint reflection coming from the front of the glass (just from the glass itself, not because of any coating). This double reflection is not what you want in a telescope, but it's fine if you're just shaving, combing your hair, or signalling airplanes.

Bottom line - unless you're building a telescope, forget about a parabolic front surface mirror. It's not what you want for emergency signalling or firestarting. Cool idea - but just not practical.

I'm still trying to figure out why the handheld parabolic dish would be any better than a simple magnifying glass - if your goal is to start a fire using the sun. Maybe the dish would work a little better on partly-cloudy days.

A few years ago I spoke to a Coast Guard helicopter pilot about emergency signals. I asked him what was the best (simple) emergency signal that a person in a kayak could use. The intention was to identify a simple, effective signalling system - if a paddler needed help a long way from shore. His reply was that signalling mirrors are still one of the best tools available. Those simple signal mirrors - that you often find in survival stores - can send a bright flash over 20 miles. Provided it's a sunny day, the helo pilots are more likely to notice that bright flash of reflected sunlight ... than anything else.

Signal mirrors are compact, inexpensive, and lightweight.

[Obviously you need something different for cloudy days, or emergencies at night].

Pete

See your point. Out as signaling device. I guess we would need to have someone with a heat temperature gun or something of the like test: a reg mirror, a fresnel, and the dish to see which one got the hottest. The dish I was looking at said something to the effect of hundreds of degrees. Or a simpler test, which started a fire the fastest.

My best shot with a signal mirror was eleven + miles, across a hazy, polluted Tucson sky. The helo pilot knew where we were before he even lifted off. We used them extensively and effectively, commonly for locating teams on the other side of a canyon, etc. I can recall at least one occasion where a party in trouble attracted needed attention through use of a signal mirror. They thought they were sending Morse code, while observers saw only disjointed mirror flashes. It was still enough to arouse curiosity.

A couple of points - it is possible to overuse them. Do not keep flashing the aircraft once they have a bead on you. It will be uncomfortable for the pilot.

Signal mirrors are great for putting light in dark places like mine shafts, rock shelters, and cave mouths. Learned this trick from mine safety officials, whose device for cave shafts was the mirror off of a standard size medicine cabinet - very illuminating!

The effectiveness depends on the size - on the amount of light it can gather and bring to a point. It's easier to make a 4.5" mirror than a 4.5" lens, because the mirror doesn't have to be transparent all the way through.

This may not make any practical difference, but if I remember my high school physics, a mirror is going to reflect visible, IR, and UV light to the same focal point, as long as it's a front surface mirror.

A lens, on the other hand, will have a different focal point for different wavelengths of light, because the refraction is a little different - chromatic aberration to the photography geeks. A glass lens is also going to block short wave UV and probably long wave IR. So the metal mirror should give more light in a smaller area, and thus more heat.

A flat fresnel lens is a lot handier than a small parabolic mirror, though.

Hi T_Co.
I have used the concave mirror from a woman's makeup kit to light a fire a few times. It was only a little bit awkward to find the focal length and start the tinder burning, but it did start a fire very quickly on a sunny day.
It was not front coated and it was smaller than 3 inches.
I doubt if being strictly parabolic instead of spherical matters much on something so small. You need to aim it a bit sideways at the tinder you are holding in your hand because if you don't you end up blocking the sun with your hand.
It was not as compact as a magnifying glass but it was in a compact.

Now you have me wondering about a ladies powder compact with the magnifying mirror as a fire starter kit. An empty compact would have space for tinder, matches, maybe a sparklite and of course a magnifying mirror in the lid. I bet a Fresnel lens would fit in it too.