TL;DR

What are some reasonable (ie: safe, portable, reusable) ways to start a fire with a magnifying glass without damaging my eyes from staring at the focused sunlight?

Background

I recently started practicing some different fire starting techniques than I usually use, including starting a fire with a magnifying glass. When starting a fire with a magnifying glass, you focus the sun's light into a small but super-bright spot on a material to heat it up.

magnifying-glass-focused-rays-burn-hole-in-paper Image Source: ScienceABC


My biggest problem when trying to light my kindling with a magnifying glass is keeping track of the blindingly bright spot that is necessarily part of this process. It is difficult for me to keep track of what I am doing since I try not to stare at the bright spot too much. Looking straight at the spot for an extended time is dangerous to your eyesight.

The last two times I tried this I have burned spots into my vision. One of the times I gave myself temporary color blindness for a while.

Per comment by fgysin, I will try to find something more than my anecdotal evidence for the danger of this.

Criteria

Now I'm worried about this and want to make sure I stay safe. Fire safety factors aside for this question; I am asking strictly about eye safety, and strictly about over-exposure to sunlight when concentrating said sunlight with a magnifying glass.

Some criteria to make a good answer:

  1. Must allow user to look at the focused spot without damaging eyes
  2. Must allow user to look at the focused spot long enough to start a fire with it (One minute is probably more than enough once I get better; less is needed in optimal conditions)
  3. Must be reusable a lot of times. If the method requires something to be continually replaced, like batteries, then I might as well just bring matches and keep replacing them
  4. Should be portable for recreational use such as while camping or hiking; the more portable the better since I would like to take it in a long-duration hiking pack.

Please keep other reasonable factors in mind, even if I have neglected to mention them.


Some ideas I had which I don't think will work

These ideas can form a baseline by demonstrating what I think is almost reasonable but not quite good enough. If you think I am wrong about any of the following, then please do make an answer refuting my worry.

  • Sunglasses
  • Welding mask
  • Mirror
  • Phone camera (or other digital camera) for indirect viewing

The sunglasses I assume are not strong enough, and after my previous two attempts I'm too scared to try it.

A welding mask would technically work but it is big and bulky and would serve no other purpose.

My welding mask does allow me to remove the small viewing "window" from it, essentially allowing me to hold up a small (approximately 2-or-3 inch by 3-or-4 inch) light-blocking lens. This is almost an answer, but my welding mask requires batteries to operate.

Likewise, other bulky eclipse-viewing devices should be considered out of scope if they are not easily portable.

A mirror just redirects the problem, I think, but maybe there is a clever way to use a small hand mirror to safely redirect the image that I'm not aware of.

I have read that some cameras can be damaged by imaging the sun if I use it a lot combined with the magnifying glass, I'm afraid it will be ruined and therefore not be a good solution.

So, does anyone have any ideas for how to do this without going blind after a few camping trips?

Experience from people who do start their camp fires with magnifying glasses is obviously great but not necessary, so please don't hesitate to pitch in with ideas you think could work. If they sound safe, I'll try to attempt them.

  • I did not include tags like camping since this can apply to multiple outdoor activities. camping, hiking, survival, etc.. – Loduwijk Oct 5 '17 at 3:07
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    Why not use eclipse glasses? They are compact, lightweight and cheap. – Qudit Oct 5 '17 at 3:53
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    Do you have a source for the bright-spot of a (small, handheld) magnifying glass being dangerous to look at? – fgysin Oct 5 '17 at 6:06
  • @fgysin I had included anecdotal evidence in my question, as I have damaged my own eyesight by using them, but Charlie edited it out in an effort to make my Q easier to read. Most of what he did helped, but that was in there specifically because I thought someone would ask the question you did. I will edit back in a shorter version. – Loduwijk Oct 5 '17 at 10:57
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    I think if you're spending that much time staring at whatever it is you're trying to burn, then you probably need better kindling. – ShemSeger Oct 5 '17 at 17:28

Use your finger.

Equally to using your finger to cover the sun when trying to spot something in the sky you can use the principle to cover the bright spot.

You will be closing one eye and putting the finger between your eye and the bright spot to cover it just enough to not bother you enter image description here

I used that system several times both with the sun but also if I had to do a couple of tack welds and someone else was using my welding mask (my face was protected and me mentioning doesn't mean I advocate this system for regular welding).

I would also suggest you use a charcloth, besides catching fire easily the general glare from a charcloth is much less than from a white piece of paper.

And yes, I lit fires with a magnifying glass but, in those cases, I never felt a particular need to protect my eyes.

In your requirements you say it must allow looking at the bright spot. Honestly, unless you are the kid burning ants with the magnifying glass, you don't really need to look right at it.

And if you are worried about looking directly at the bright spot then don't consider a mirror, you wouldn't want to reflect the light right in your eyes by some freak accident.

I would just use eclipse glasses. As they are safe to look directly at the sun with, then they should be fine to look at the bright spot caused by the magnifying glass.

Just make certain that they are certified and not rip-offs and then you should be good to go. They meet all of your requirements and would be no more trouble to take with you than a pair of sunglasses which you probably already have.

  • The eclipse sunglasses might be too shady, but I will try it and find out. My welding mask shading is adjustable, and when I had it turned up high enough that I could look directly at the sun, I could not see anything else; all else was completely black. If I want to be able to work with something, I would need to turn its shading waaay down, which I don't think eclipse glasses can do. I will buy a pair, try them out, and report back, but that might take some time. – Loduwijk Oct 5 '17 at 11:10

One option would be to use snow glasses. They've been used for centuries to prevent snow blindness, and work by limiting the amount of light that can enter your eyes. They are simple to make, and can be constructed out of any opaque material you can put an eye slit in. To control the amount of light entering your eyes, simply make the slit smaller.

These would be an excellent option because you wouldn't have to carry anything extra with you, they're traditionally made out of wood or tree bark. We used to make them as kids in school up here in Canada out of a chunk of cereal box, but you can also make a simple pair out of duck tape.

In a pinch, you can just look through the tight gaps in your fingers, which may be sufficient for your needs, just cover your face with your hand and peek out a tiny slit between you fingers. I do this quite often when stepping outside for a moment from the dark indoors into the blinding white of a bright snowy day.

enter image description here enter image description here

Use a non-battery welding mask glass. You can carry it in the same pouch as your magnifying glass. One can be Purchased new for a couple of dollars. They can also be had second hand.

  • 1
    This is a very workable solution, but a slightly more elegant solution is available: glasses for oxy-acetylene welding. They are shaped like regular glasses, made of plastic, fairly cheap, and available in (at least) shade #5. This is much darker than regular sunglasses but you can still see your hands under bright sunlight. Many choices on Amazon, and probably your local welding supply. – Pepi Oct 10 '17 at 8:36

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