Fresnel lens magnifier
Starting a fire is one of those manly skills you just gotta learn. Anyone can start one with a pack of matches or a lighter, but what if you don’t have any or you run out or lose them somehow? Starting a fire without matches is pretty easy if you have a little knowledge.
There are actually quite a few ways to start a fire without matches or a lighter. One way that can work is by using a magnifying glass. We’ll go over that first, and then I’ll tell you a way that may be a better choice for a bug out bag.
Starting a fire with a magnifying glass
So first, how do you start a fire with a magnifying glass? The concept is pretty simple. The sun has a lot of energy but because it’s so far away, that energy is spread out in all directions. A tiny fraction of that energy hits the earth (something like 0.0000000454 percent hits the upper atmosphere and then less than half of that reaches the ground). Because it’s so diluted, that light energy isn’t enough on the Earth to catch stuff on fire. The amount of energy per square foot hitting something on the Earth isn’t all that much – about 100 watts for a square foot. 100 Watts on a surface that large can make it hot, especially if it’s black, but won’t light. What you need to do is focus all that energy into a smaller spot.
What a convex lens will do is take that light energy and focus it into a smaller area so the energy is focused and the temperature will get hot enough to light: about 451 degrees (which is where they got the name for that book Fahrenheit 451). That means that if you had a magnifying glass that was a square foot of surface, you could focus about 100 Watts onto one spot, and easily reach past 451 degrees. That will definitely catch something like twigs, leaves, or paper on fire, especially something like dry tinder. The problem is that a square foot of lens is about 14 inches in diameter (Area = pi * radius squared) so you can’t lug it around easily, especially if you want one that has a short enough focal length to work really well.
Now obviously, you don’t really need 100 Watts of energy on a spot to start a fire but you don’t want to go too small either because in real life, the Watts hitting your tinder are gonna be even less due to clouds, smog, the season changing the angle of the sun, how clear your lens is etc. That’s why the smaller lenses don’t always work in real life. In a survival situation, you need it to work.
The image above is from a biconvex lens, meaning it’s convex on both sides, and convex means it’s bulged out. The bulge affects that focal length, which determines how far the thing you’re gonna burn needs to be from the lens to catch fire, it determines the size of the area that the energy is focused onto (meaning how hot it is).
Because a large lens isn’t feasible to carry, your choice is to go with a smaller lens (which means less light energy will go to starting your fire) or using something called a Fresnel Lens.
What is a Fresnel Lens?
So the nice thing is that some dude named , born in the 1700’s, figured out how to take a and flatten it to a sheet so it could be used in lighthouses. He figured that he could cut the lens into concentric circles and then flatten them down by laying them inside each other. Theoretically, if you had an infinite amount of circles, you’d come close to the same efficiency as the convex lens. Unfortunately, that’s not possible so Fresnel lenses will always be less efficient than a normal magnifying glass. The nice thing though is that they’re essentially flat and can even be flexible.
Fresnel lens vs plano convex lens
So how inefficient are they and about how much energy could you expect under perfect conditions?
An 8 1/2 by 11 sheet of paper is about .649 square feet. That would be about 65 Watts hitting the sheet that you could focus onto an area. Fresnel lenses aren’t nearly as efficient as a normal convex lens though so you have to factor in some loss. Different types of Fresnel lenses have different losses, and I’m not gonna go all into those details because it really doesn’t matter. Check out this if you want more info.
We’re just gonna pick a typical range and use 70% as the efficiency of the lens. 70% of 65 Watts is 45 Watts – about the same amount as a 9 inch lens will focus (depending on the focal length). That means that you it’s possible to carry a flat sheet of flexible Fresnel lens and have about the same energy to use for starting a fire as a 9 inch convex lens.
The very cool thing is that Fresnel lenses of this size are really light and take up almost no space in your backpack. You could just slide it in where your water pack goes or inside against your back. Also, they’re hella cheap. I picked up a 
Think it doesn’t work? Watch this video I made today about using the Fresnel lens I bought to light some leaves I piled up in the back yard. This was in pretty much full sunlight but there were clouds out. It works better if you grab a handful of dry leaves and crush them like in the video…
Once I got the light onto the leaves, it took about 3 seconds to flame up. You just move the fresnel sheet closer and farther from what you’re trying to light until the focused light spot is smallest, and then flex the sheet to make it as small as you can get it. The smaller the focused light is, the higher the density of light there is, so the hotter it will get. You’d want to have your kindling and the rest of your wood close by so it could catch before the leaves go out. In some cases, you’ll want to move the light around a bit to catch more of the leaves on fire, especially if you have clouds coming.
So what if you wanted to man up and get a bigger lens for your home or RV? A projection TV usually has a Fresnel lens as one of their front screens. If you found one of these TV’s on Craigslist or at Goodwill that didn’t work, you could pull out the lens and make one hell of a solar burner thing for things like cooking, sterilizing water or making some kind of field-expedient death ray. Watch what this guy fried with one of these TV screen Fresnel lenses he pulled out…
This is the Fresnel lens I got
So if you want a super-cheap, badass way to start a fire on your next camping trip or while living in the woods while avoiding an alien hoarde, just pick up one of these babies.
– Syndicated from
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