# Fun with Fresnel Lenses... **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [May 5, 2006, 2:53pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541 "2006-05-05T14:53:00Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![MacTech](https://avatars.discourse-cdn.com/v4/letter/m/c0e974/32.png) [@MacTech](https://boards.straightdope.com/u/MacTech)\ **Post date:** [May 5, 2006, 2:53pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/1 "2006-05-05T14:53:00Z")
A few weeks ago I picked up one of those “Mini-survival kits” with the spark-lite firestarter, signal mirror, whistle, fishing line, compass, etc… on a whim (okay, okay, I wanted the Spark-Lite firestarter, is that so wrong? ;)), primarily to put it in my car’s center console, for “just in case” type situations… It came with two firestarting devices, the aforementioned “Spark-Lite” (basically the spark wheel and flint of a disposable lighter in a plastic handle) and wax/cotton fiber tinder, and a fresnel lens… I’ve never really played with fresnel lenses (beyond using them as a magnifier lens to look at small stuff) in a firestarting capacity, but burning stuff using just the power of the sun is _always_ fun… 😉 anyway, I decided to do a comparison between my standard magnifying lens (the lens in a Victorinox Swisschamp Swiss Army Knife) and the Fresnel… The Fresnel had the size advantage right away, it was 4 times larger than the dinky lens in the SAK, so it wasn’t strictly a fair comparison First things first, safety is paramount when you’re attempting to combust materials using solar radiation, so I went to the local hardware store and picked up a pair of welding goggles, their dark, dark green lenses protect my eyes from the focused spot of the sun (and they give me that endearing “Mad Scientist” look to boot, FOOLS, I’LL DESTROY YOU ALL, THEY LAUGHED AT ME, WELL, SOON IT WILL BE _ME_ WHO’s…err, sorry, got caught up in the moment there…) I grabbed a small piece of dry wood and focused the SAK lens, it took at least 45-60 seconds for the wood to start darkening and smoking… Using the Fresnel, it took about 10 seconds, and the spot of carbonization was at least twice as big as the SAK spot I grabbed a cigarrette butt that still had some tobacco left on it, attempted to relight it with the SAK lens, no luck, just didn’t have the “oomph”, however the Fresnel re-lit it in about 20 seconds, I even burned a hole in the filter with it… Clearly the credit-card sized fresnel lens was the superior burner So last night, I stopped off at the local bookstore and picked up 3 of the flat book magnifier lenses (these are also fresnel lenses) 2 4X6" lenses (one for the survival kit, one just to play around with) and one of the credit-card sized ones to put in my wallet, so I _always_ have the ability to burn stuff using the sun… (hehe, burning stuff is cool, yeah, yeah, hehe) this morning, I decided to put the 4X6" lens to the test, I put a _WET_ magnolia leaf on the driveway and focused the lens to a spot, within THREE seconds, the leaf began to smoke and blacken, remember, this was a _wet_ leaf… I then grabbed a plastic bottlecap from a 20 Oz soda bottle (dark blue bottlecap), and focused the lens on the top, the plastic began to blister, smoke, and discolor after 5 seconds, and after 30-45 seconds I had burned a pencil-diameter hole through the bottlecap _and_ the plastic liner underneath On a whim, I grabbed a blank CD-R, and tried to burn that as well, that one _didn’t_ work, I stood there for at least 2 minutes, nothing, clearly the focused sunspot wasn’t the right frequency of light/intensity/something, perhaps the reflectivity of the CD-R worked against it… Still, that 4X6 Fresnel lens is an impressive little burner… If the weather’s nice this weekend, I may have to perform some more… “research”, yeah, that’s the ticket, research… ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [May 5, 2006, 2:56pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/2 "2006-05-05T14:56:04Z")
The true test of lens burning is ants. Will it burn ants? ---
**Author:** ![lieu](https://avatars.discourse-cdn.com/v4/letter/l/c2a13f/32.png) [@lieu](https://boards.straightdope.com/u/lieu)\ **Post date:** [May 5, 2006, 2:58pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/3 "2006-05-05T14:58:52Z")
Fire ants? Apparently. ---
**Author:** ![Spatial\_Rift\_47](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Spatial\_Rift\_47](https://boards.straightdope.com/u/Spatial_Rift_47)\ **Post date:** [May 5, 2006, 2:58pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/4 "2006-05-05T14:58:56Z")
How about dogs? You burning your dog? ---
**Author:** ![lieu](https://avatars.discourse-cdn.com/v4/letter/l/c2a13f/32.png) [@lieu](https://boards.straightdope.com/u/lieu)\ **Post date:** [May 5, 2006, 3:01pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/5 "2006-05-05T15:01:14Z")
[David Byrne] I burning down… you know. [/David Byrne] ---
**Author:** ![Tikki](https://avatars.discourse-cdn.com/v4/letter/t/5daacb/32.png) [@Tikki](https://boards.straightdope.com/u/Tikki)\ **Post date:** [May 5, 2006, 8:26pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/6 "2006-05-05T20:26:02Z")
Dang! You made me get out my fresnel lens out of my sewing kit and run outside with a scrap of paper, all for naught. The lens wouldn’t focus the sunlight tight enough to even produce a slight discoloration. I’m so disappointed. I wanted to burn–I had the _need_ to burn–but the credit card-sized lens let me down, man! That sure was a helluva bright light it made though. I definitely recommend some kind of eye protection. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [May 5, 2006, 8:35pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/7 "2006-05-05T20:35:23Z")
Burning ants with Fresnel Lenses – you people are sick. > [@](#): > > That sure was a helluva bright light it made though. I definitely recommend some kind of eye protection. You actually can use focused sunlight for woodburning. They wrote it up in an old 1960’s issue of **Boy’s Life** magazine. But they strongly recommended wearing very dark sunglasses because of the glare. By the way, if you have one of those flexible card-sized Fresnel Lenses, you can try roating it or flexing it a bit to finesse the concentration of light. Odds are you can only tigfhten it up along one axis, giving you a line instead of a blob, but that might be good enough. better still is to get a well-corrected rigid lens. Glass will probably do better that plastic. Plastic _can_ be made into a good lens, but only if it’s processed right. And really cheap plastic lenses usually aren’t made right. ---
**Author:** ![MacTech](https://avatars.discourse-cdn.com/v4/letter/m/c0e974/32.png) [@MacTech](https://boards.straightdope.com/u/MacTech)\ **Post date:** [May 5, 2006, 8:48pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/8 "2006-05-05T20:48:31Z")
Further test results; black plastic Powerade sports drink cap; instant discoloration as soon as the focused spot of light would hit it, started smoking in 5 seconds, in 10 seconds there was a smoking pit of _BOILING_ plastic, first time i’ve ever seen plastic _boil_ pile of dust and lint removed from the inside of an iMac G4 and stuck on a piece of packing tape to prevent it from blowing away; _instant_ smoke, blackened and carbonized in 5 seconds, yet no flame… old dry piece of wood; smoke in 3 seconds, deep hole burned in the wood in 15 seconds, no flame though old _WET_ piece of wood, i mean _sopping_ wet; smoke in 10 seconds, hole in 30 seconds, entire piece of wood (about 1" long) _dry_ in 60 seconds still to try; old (1976 minting) copper penny and new (2005 minting) copper/zinc penny ---
**Author:** ![Unintentionally\_Blank](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unintentionally_blank/32/5418_2.png) [@Unintentionally\_Blank](https://boards.straightdope.com/u/Unintentionally_Blank)\ **Post date:** [May 5, 2006, 8:49pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/9 "2006-05-05T20:49:47Z")
When we moved, we paid for professional movers so that the TV (the first thing we ever bought as a couple. Nothing says commitment like a 48 inch Toshiba rear projection TeeVee in genuwine wood grain) wouldn’t be damaged in the move. The contract labor did a GRAND job of putting a huge crease in the center of the screen. Moving Bill: $1200, TV repair bill billed to moving company $900. But the TV repair guy left a copy of the factory adjustment manual…and the damaged frensel lens. The Lens is (does the math) 28 by 38 inches. Bow to my unlimited burninating power! Now, go stand over there…a little to the left…and could you sit down? Damn, just wait there til the sun comes out from behind the cloud. Actually, it sits behind my desk in my office. Woe is the day my boys find it and decide to burn down Denver. ---
**Author:** ![MacTech](https://avatars.discourse-cdn.com/v4/letter/m/c0e974/32.png) [@MacTech](https://boards.straightdope.com/u/MacTech)\ **Post date:** [May 5, 2006, 8:53pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/10 "2006-05-05T20:53:30Z")
Hmm, perhaps i should try cooking a hamburger with the Fresnel lens this weekend… ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [May 5, 2006, 9:36pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/11 "2006-05-05T21:36:48Z")
> [@Unintentionally Blank](#): > > The Lens is (does the math) 28 by 38 inches. Bow to my unlimited burninating power! Now, go stand over there…a little to the left…and could you sit down? Damn, just wait there til the sun comes out from behind the cloud. > > Actually, it sits behind my desk in my office. Woe is the day my boys find it and decide to burn down Denver. I guess you need to worry about them burninating the countryside and/or peasants. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 5, 2006, 9:43pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/12 "2006-05-05T21:43:31Z")
> [@Unintentionally Blank](#): > > The Lens is (does the math) 28 by 38 inches. Bow to my unlimited burninating power! Now, go stand over there…a little to the left…and could you sit down? Damn, just wait there til the sun comes out from behind the cloud. > > Actually, it sits behind my desk in my office. Woe is the day my boys find it and decide to burn down Denver. No thatched-roof cottages are safe. ---
**Author:** ![threemae](https://avatars.discourse-cdn.com/v4/letter/t/5f9b8f/32.png) [@threemae](https://boards.straightdope.com/u/threemae)\ **Post date:** [May 6, 2006, 1:27am UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/13 "2006-05-06T01:27:13Z")
But from a survival standpoint, the real question is whether any of you have actually managed to start a fire. Second, in what survival situation would you actually need a fire if you had bright sunshine with which you can actually start a fire? ---
**Author:** ![Tikki](https://avatars.discourse-cdn.com/v4/letter/t/5daacb/32.png) [@Tikki](https://boards.straightdope.com/u/Tikki)\ **Post date:** [May 6, 2006, 2:18am UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/14 "2006-05-06T02:18:22Z")
> [@CalMeacham](#): > > You actually can use focused sunlight for woodburning. They wrote it up in an old 1960’s issue of **Boy’s Life** magazine. But they strongly recommended wearing very dark sunglasses because of the glare. **The Mother Earth News** had an article about this too. The authors actually had a double lens setup that I think came from a lighthouse. If I recall correctly, it was once the rotating part behind the Fresnel lens. Or it might have had something to do with some old movie equipment. Anyway, after I read about it, I went out and did my own woodburning with a regular magnifying lens. Took a square of pine board and burned a scene with pine trees and a mountain. Painted the edges blue, signed and dated it and then varnished it. It’s amateurish but to this day it still stands on the back of my stove. I just checked the date on it and found it’s 26 years old. Co-incedentally, the magifying lens I would have used today if I’d known which of my cluttered drawers I’d last seen it in was the one I used then. ---
**Author:** ![Ale](https://avatars.discourse-cdn.com/v4/letter/a/f19dbf/32.png) [@Ale](https://boards.straightdope.com/u/Ale)\ **Post date:** [May 6, 2006, 2:33am UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/15 "2006-05-06T02:33:54Z")
> [@Unintentionally Blank](#): > > The Lens is (does the math) 28 by 38 inches. Bow to my unlimited burninating power! Now, go stand over there…a little to the left…and could you sit down? Damn, just wait there til the sun comes out from behind the cloud. Can it burn Roman ships at a bow-shot distance? ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [May 6, 2006, 2:34am UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/16 "2006-05-06T02:34:40Z")
> [@Spatial Rift 47](#): > > How about dogs? You burning your dog? Use this one to burn your dog. \*\* Big Fresnel Lenses \*\* 395mm x 395mm Item: #A395b Thickness: 2mm Weight: 350 grams Material: optical acrylic Groove pitch: 0.5mm Focal length: 220mm PS I’m calling SPCA now. ---
**Author:** ![drewbert](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drewbert/32/15890_2.png) [@drewbert](https://boards.straightdope.com/u/drewbert)\ **Post date:** [May 6, 2006, 2:43am UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/17 "2006-05-06T02:43:12Z")
Mere amateurs, all of you. [http://www.solardeathray.com/](http://www.solardeathray.com/) ---
**Author:** ![Tomcat](https://avatars.discourse-cdn.com/v4/letter/t/8e8cbc/32.png) [@Tomcat](https://boards.straightdope.com/u/Tomcat)\ **Post date:** [May 6, 2006, 8:17am UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/18 "2006-05-06T08:17:56Z")
What, exactly, do “Survival” firestarters have over a common cigarette lighter? Heavy magnesium scrape blocks, sparkers and fresnel lenses all seem like neat toys, but, really, isn’t a cigarette lighter more reliable, effective and cheaper? -Tcat ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 6, 2006, 1:34pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/19 "2006-05-06T13:34:35Z")
> [@Tomcat](#): > > What, exactly, do “Survival” firestarters have over a common cigarette lighter? Heavy magnesium scrape blocks, sparkers and fresnel lenses all seem like neat toys, but, really, isn’t a cigarette lighter more reliable, effective and cheaper? > > -Tcat I’ve had the same thoughts. I’ve always thought it was neat that you can make a fire with steel wool and a 9v battery. There’s a great survival tip for you! Only who’s going to carry around a wad of steel wool and a 9v battery, when he can just put a disposable lighter in his pocket? Or pockets, since it would be a bad idea to keep them in the same one. But then I have a few magnesium firestarters. Mangesium firestarters aren’t heavy. There is a very small chance that a disposable lighter can leak, especially if you’re in a crash. A Zippo will dry out before you use it. Matches can get wet. So a magnesium firestarter is something that will not leak, will work when wet, and would be a good thing to have in a desperate situation. And using magnesium, which burns very hot, may allow you to ignite damp fuel better than a lighter. Certainly better than rubbing two sticks together. Only I’ll still flick my Bic for as long as the fuel holds out. ---
**Author:** ![Tomcat](https://avatars.discourse-cdn.com/v4/letter/t/8e8cbc/32.png) [@Tomcat](https://boards.straightdope.com/u/Tomcat)\ **Post date:** [May 6, 2006, 2:18pm UTC](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541/20 "2006-05-06T14:18:24Z")
Yeah, I’m just playing to the odds. I have Bics in first-aid kits, in backpacks, in pouches, in jackets, in pockets, etc. $0.50 a piece and I have multiple backups. Plus, a magnesium block generally works better with a knife. So now you need two things to start a fire, not just one. Again, neat toys, but…I am going to make the declaration that the best firestarter in the world in terms of reliability, price, ease of use and more is a Bic. Plus it serves as a light source, hand warmer, tool (sealing rope ends, etc) and is also refillable. Gotta love the Bic. One of my favorite stage jokes involves lighters. I forget who did it first, but if you are giving a speech or doing a magic show or ??? This is funny as hell: Buy a whole bunch (20?)of lighters and put them into a (out of sight) clear bag or jar or ??? When on stage ask for a plastic lighter from the audience - try and get as many as you can from them. They will throw them to you. Take them and add them to the jar or bag and say “That works every time.” or “Doing my bit to end lung cancer.” and don’t return them. 99% of the audience will erupt in laughter. -Tcat [Next page](https://boards.straightdope.com/t/fun-with-fresnel-lenses/355541.md?page=2)