Make Fire w/ Fresnel Lens?

[m801]

In some of the survival kits
there is a 3" x 2" fresnel lens;
to be used to start a fire.

Has anyone used a 3" x 2" fresnel lens to start a fire?

Is this a worthwhile method of fire starting?

 

[ofuzzy1]

Yes! Many times.

Go to your $ store and buy a big 8 x 11 for even better results!

 

[BH]

Never used a fresnel lense, but have used camera lenses quite a few times. A good quality hand lense (like jewlers and geologist use) will also do the trick and is small and easily worn around your neck. Don't know about now, but back in my geology student days (70's) they cost about $30.

 

[LoupGarou]

Yep, keep two of the credit card sized ones in my wallet and a 12"x12" one in each car (removed from overhead projectors). I also have two 47"x47" ones that Edmund Scientific used to sell. The 12" ones will start a good fire quickly with steel wool, dryer lint, char cloth, toilet paper or even just newspaper. The will also melt solder (and lead) into molds. The bigger 47" lens will melt copper pipe and quite a few other metals, as well as make concrete explode.

If you want lab spec units, Edmund Scientific sells quite a few different sizes. If you don't mind using experimenter grade (perfect for this type of use) lenses, I would suggest their outlet "sister company" that has taken over their old manufacturing plant in Barrington, NJ called Anchor Optical Surplus (which BTW is a good place to visit every once and a while if you are ever in the area!). The website is www.anchoroptical.com .

They have the 12" (11.25") ones (16" focal length is the one you want, B70877) for $5, and a larger one (25"x33" with a 60 inch focal length, B32736) for $45.

And with the 60" focal length, you don't even have to bend over to light the fire. Just make sure that your target is not your neighbors foot before you start focusing in...

Loup Garou

 

[Walrus Whisperer]

And watch out you dont accidently shoot a beam at a person or yourself in the head area-you could burn a retina with those things. Its even hard to look at the spot on the ground its so bright....
I got to build a small frame for mine-its one of those RV things that sticks on the back window so they can see behind them and its a floppy plastic thats hard to handle-found it for 2$ at a yard sale.

 

[LoupGarou]

And watch out you dont accidently shoot a beam at a person or yourself in the head area-you could burn a retina with those things. Its even hard to look at the spot on the ground its so bright....
I got to build a small frame for mine-its one of those RV things that sticks on the back window so they can see behind them and its a floppy plastic thats hard to handle-found it for 2$ at a yard sale.

The good thing is that usually the focal point is below the lens, so a direct "into the eye" shot is hard to do unless you are playing with the lens, looking up through it. As far as looking at the spot, keep a piece or two of exposed color negative film (35mm) handy to use as a viewer (it also makes a good Near IR pass filter). The other option is asking your optometrist for a pair of the sunblockers that they give out when they dialate your eyes.

Loup Garou

 

[LilRose8]

Could you use this to heat soup or water if you didn't want to start a fire?

 

[LoupGarou]

Could you use this to heat soup or water if you didn't want to start a fire?

The bigger ones can be used to heat things if you keep the focus off (don't let it make a point focus), and you hit a dark colored target. The problem with using a lens to heat things is that you have to hold the lens or make a holder to keep it on target. Edmund used to sell a frame, which I have seen plans for around the net. It keeps the lens held toward the sun, long enough for it to do some work. You may have to move either the target or the holder every few minutes since the earth rotates and the focal point "moves" off of the target. I would try a frame and holder with the 25"x33" lens focused down to a 4" spot. That should give you about 50 times the heating capacity of the straight sun and cook pretty well. You could even set the pot on a pyrex plate and use a mirror to reflect the beam (well before the focal point), up onto the bottom of the pot, evening the cooking heat out instead of putting it on one side. And a 4 inch focal point would work good with most camp pans/pots. I would try to find one with an aluminum or copper base disk to spread the heat out anyway.

While slower, the mirror based solar cookers/ovens work good because if made right, they will heat for an hour or two while you do something else.

Loup Garou

 

[Walrus Whisperer]

Ohhhhh yeeeaaahhhh!
See:
http://www.armory.com/~images/?s=solsticeCook2001

Also just do a seach for "cooking with fresnel lenses", tons of stuff....

 

[LoupGarou]

Another good source of solar cooking fun is an un-needed DSS satellite dish. Remove the feedhorn, but not the arm that held it. Take fine sandpaper and sandpaper the inside surface of the dish till it is shiny (not mirror finish, just metal with some very small lumps still in the surface). Once this is done, find a can of rustoleum gold metal spray paint. Real gold is a very good reflector of both near and far(heat) infrared. The paint does help over just plain steel color. Once the gold paint dries, coat it with a layer of clearcoat to keep the dirt down. Fix a holder to the arm so that it will hold a dark color painted soup can near the same position that the feedhorn was held at (the translucent white part of the feedhorn is near the focal point). Aim the dish at the sun till you see the spot appear on the backside of the can. You will have to move the dish every few minutes to keep the spot on target, but it cooks pretty fast. Flat dark blue or flat black works the best for the target.

Don't use the older 8 or 10 foot dishes (or larger), they can melt the can. Very quickly if focused too.

Loup Garou

 

[Splicer205]

When DS was little he left a magnifying glass in the car on top of a piece of paper. It burned a hole in the paper beneath it, and was smoldering when I found it. He was quite taken with the phenomena.

 

[Chronicles]

I use to be a TV and electronics service technician and one really cool fresnel lense can be found in a Big Screen TV, the rear projection style. These are in junk piles now, but if you pull apart the linticular away from the lense, which are taped together. You will be able to set ablaze a brush pile in just a few seconds.

This is the front screen area that can be easily removed from a big sreen TV. They are around 4 foot square..

If say you had more than one and mounted, for control,, then, you could track the sun, you could boil water or create much heat at a focal point. Just thought I would let you folks know about these..

 

[ofuzzy1]

Ed & Walrus thanks for the links.

www.alltronics.com has these:
http://www.alltronics.com/lenses.htm
Giant Fresnel Lens
Build a solar furnace with one of our giant Fresnel lenses the size is approx 31" x 41".
The material is a plastic sheet approx 1/16" thick.
Made by Kuraray P/N 491P043A90
Shipped via regular UPS but do to dimensions it will be a flat $28.00 for shipping to the Continental U.S.
22L004 price $89.95 each

Qty:

Giant Fresnel Lens
Build a solar furnace with one of our giant Fresnel lenses the size is approx 37" x 28".
The material is a plastic sheet approx 1/16" thick.
Made by Kuraray P/N 491P044A60
Shipped via regular UPS but do to dimensions it will be a flat $28.00 for shipping to the Continental U.S.
22L005 price $79.95 each

 

[m801]

dudes I know the larger sizes are near death-ray kindsa things,
but I was speaking to the credit cards size fresnels
that are showing up in survival kits.

if they aren't up to really starting fires then I won't put them in my family's survkits

all input is appreciated - thanks

 

[gillmanNSF]

Dude, you say deathray like it's a bad thing. To ants, those pocket-sized lenses are rays of death! Alas, I'm reformed, but it don't take much to stir up that old kid inside.



Now back to the gargantuan deathrays and the damage that we can do.....







I wonder if they can be used to blind an enemy?

 

[Moondoggie]

Blind? pfffff. More like boil their eyeballs out. as long as their about a foot away.

We need to build some HUGE ones and take em to Mars an put em in orbit to heat up the planet.

[edited to add]

I've started several fires using the small credit card size ones. They work better the standard magnifying glasses.

Moondoggie

 

[jed turtle]

giant fresnel lenses. killer beams. hmmm. i'm getting visions of tilting mirrors at the focus, aiming the beam somewhere else... lighting up approaching enemy ships?

seems like you'd need to put another lense there first to straighten out the converging beams and restore them to parallel paths before letting them hit the tilting mirror. anyone got thoughts about that?

 

[LoupGarou]

dudes I know the larger sizes are near death-ray kindsa things,
but I was speaking to the credit cards size fresnels
that are showing up in survival kits.

if they aren't up to really starting fires then I won't put them in my family's survkits

all input is appreciated - thanks

Anchor Optical also sells the smaller 3"x5" ones for $1.75. These, as well as their smaller credit card sized cousins will start a fire when needed (except at night or when cloudy, so have a backup or plan ahead). The combustible sources I mentioned earlier just make it easier to start (charcloth, dryer lint, steel wool, etc.).

And it is no a death ray, the focus is a set distance and once it passes that distance, the beam starts to spread back out. That is unless you match it with a DCV (double concave) lens of the same focal distance to make a collimator (wide beam in, narrow beam out). Then all you need is a gold plated, first surface water cooled mirror to direct the beam. Four servos and a microprocessor to track the sun, and you are all set. (Insert Evil Mad Scientist Laughter Here).

Loup Garou

 

[ofuzzy1]

Lou: Thanks --- Bwahahaha.

You can buy small lens from the office suppy places and even the dollar store.
An 8x11 is in the BOB.
And a business card sized one is in my wallet, keep it in a plastic sheath, otherwise it will get scratched, grungy and won't work as well.

 

[Ozarkian]

I have incinerated millions of "Army Ants" with a small 3 inch magnifying glass. Plus I set our front yard on fire one winter when I was a kid. I never knew short winter grass burned so good. My cousin and I got it put out after it had burned about a 20 foot diameter circle. heh. I was trying to light some old rotten wood and had left it without knowing there was smoldering cinders still going on. My Dad didn't think it was so funny and took my glass away!

 

[timbo]

My little green plastic soldiers didnt like me either. The ants boycotted our yard.

Little picket signs.........it was embarassing.