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Things You Can Do With A Giant Fresnel Lens 469

Posted by michael
from the toasty dept.
Ant writes "Here is a link where this guy always wanted Edmund Scientific's Giant Fresnel Lens. 'Melts asphalt in seconds!' the ad said. When he went to graduate school he met several other people with the same enthusiasm for aimless destruction through bizarre means, and just enough combined cash to make it happen. Thus the reign of terror began."
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Things You Can Do With A Giant Fresnel Lens

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  • EEK! (Score:5, Funny)

    by intekra (754612) on Friday May 21, 2004 @09:17PM (#9222067) Homepage
    Lets hope this doesn't get into the wrong hands! This can be worse than WMD's!
  • Burnination (Score:5, Funny)

    by OneFootIn (696599) on Friday May 21, 2004 @09:18PM (#9222071)
    And to think, when I was a kid I had to settle for burning ants with a magnifying glass.
    • by edoc (772148)
      This is what happens when you don't grow out of those times! On a positive note at least you have some change of getting laid.
    • by bergeron76 (176351) * on Friday May 21, 2004 @11:23PM (#9222775)
      Heh! When I was in High-school we took things a step farther by taking Mr. Bouknecht's overhead projector lens out of the housing. [I'm quite certain] it's a true Fresnel Lens - but the point is that it was like a magnifying glass on steroids. We had to be careful just walking out into the parking lot with it because if it caught a friends leg and the sun at the right angle, they'd be in a world of hurt.

      Aaahh, fun times being a juvenille. My advice to the next generation though - be as mischievious as normal, but don't do anything stupid or anything that will get you thrown in jail. Be intelligent with your "inevitable" delinquency [you call it rebellion] - we've all been there - however don't get anyone hurt or needlessly damage property.

      Life is much more exciting as an experienced adult making thousands of dollars a month. Trust me on this one all you guys. Science is fun, but it can also be very dangerous... Be safe - because if you go to jail or die, the guy you hate will likely reproduce and you never will. And that would suck.

      • by Skjellifetti (561341) on Saturday May 22, 2004 @01:15AM (#9223249) Journal
        Science is fun, but it can also be very dangerous... Be safe

        My father once caught me making gunpowder using a chem lab kit he'd bought me and an old mortar I'd found in the garage. His first reaction was "Idiot kid!" Then it was "Oh, wait, that's what I used that mortar for when I was a kid. Let me show you how to do it safely." Basically wet the ingredients down so it doesn't accidentally catch fire while you are grinding it. I suspect he'd learned that one the hard way. I'm big on the safety angle with my kids now, too. In my case its a result of an experiment with some gasoline and an Estes rocket engine when I was a teenager.
      • by Thing 1 (178996)
        [...] however don't get anyone hurt or needlessly damage property.

        I'd love to see the attempt at explaining to the cops how you needfully damaged that property...

    • by Tablizer (95088)
      when I was a kid I had to settle for burning an ant

      Now you can burn like......two ants!
  • by PissingInTheWind (573929) on Friday May 21, 2004 @09:18PM (#9222076)
    Right here: Cooking with Light [armory.com].
    • Seeing as how the person who did this first works for SCO [armory.com], should I be concerned if I want to copy his idea? :)

      Though seeing as how he lets people use it at the burning man festival, he doesn't quite seem like the type to do that... sounds like a very neat guy.
      • He in fact is a very neat guy. He is typical of the old school Santa Cruz geek, which is to say he is atypical and good-natured and -hearted. At least, far as I can tell.

        I've known or been around a number of geeks who worked for SCO in the old days, and they were all really cool, knowledgeable people. And, I've been sloshed at the Armory a few times. :)

    • I wonder if it would be possible to have a reflector added in to that, so that you can heat the bottom of the frying pan, for a nice even heat. I'm guessing that most mirrors would absorb too much energy themselves, though, and heat up too much.
  • Ants (Score:3, Funny)

    by eightball01 (646950) on Friday May 21, 2004 @09:19PM (#9222077)
    Now the ants really don't stand a chance!!!
  • Ideas (Score:3, Interesting)

    by panxerox (575545) * on Friday May 21, 2004 @09:19PM (#9222080)
    Mount it in front of your monitor for a really big image Write your name in the side of someone's car Wipe your harddrive permanently There has to be a way to increase solar cell output with these (not at direct focus of course mabey larger area at 25% focus)
    • Re:Ideas (Score:3, Interesting)

      by DoraLives (622001)
      Wipe your harddrive permanently

      I daresay this may be the long sought for absolutely sure method for permanently removing data from a hard drive in such a way that nobody, not even the NSA will be able to recover it.

      Job well done, guys!

      • Re:Ideas (Score:3, Interesting)

        by cmacb (547347)
        Actually the last time I opened up a hard drive I noticed the recording surface was no longer the dull iron oxide I remember from long ago, or even the shinier material they used later but something that looked more like a mirror. It would probably be a mistake to aim a large lense at one of these.
        • Re:Ideas (Score:3, Interesting)

          by dmaxwell (43234)
          but something that looked more like a mirror. It would probably be a mistake to aim a large lense at one of these.

          No biggie. Just make some big spots on the platter with a Sharpie. Once a spot gets white hot, the shininess of surrounding material won't matter much.
    • Re:Ideas (Score:5, Informative)

      by edheler (715806) on Friday May 21, 2004 @10:14PM (#9222381)
      Here [mdatechnology.net] are a few [nasa.gov] references [photonics.com] to increasing solar cell output with Fresnel lenses. Enjoy!
      • Re:Ideas (Score:5, Interesting)

        by uberdave (526529) on Friday May 21, 2004 @10:55PM (#9222608) Homepage
        From one of the articles: "Cylindrical Fresnel lenses provide a 7:1 concentration, allowing a single multijunction GaInP2/GaAs/Ge cell to collect solar energy equivalent to that gathered by seven cells."

        In other words, a fresnel lens does not help in terms of energy gathering. On a cost or mass per area, it does.
  • A good use (Score:3, Interesting)

    by KoriaDesevis (781774) <(moc.oohay) (ta) (sivesedairok)> on Friday May 21, 2004 @09:19PM (#9222082) Journal

    Maybe not very practical, but it might make a good paint remover. I have seen work crews remove paint from wood surfaces with a heat gun that looked like a big hairdryer, so I would think this type of lens would be helpful for stripping paint off metal surfaces such as water towers and so forth.

  • by Mz6 (741941) * on Friday May 21, 2004 @09:19PM (#9222084) Journal
    I wonder what his Terrorist Quotient is? [slashdot.org].
  • by ErichTheWebGuy (745925) on Friday May 21, 2004 @09:20PM (#9222086) Homepage
    A friend and I were discussing what to do with all of our AOL CD's. We both came up with the idea that you could make a similar device out of them.

    Welcome! You've got fire!
  • by Fiz Ocelot (642698) <{baelzharon} {at} {gmail.com}> on Friday May 21, 2004 @09:20PM (#9222087)
    Sharks with...giant Frensel Lenses attached to their heads!
  • by john82 (68332) on Friday May 21, 2004 @09:23PM (#9222098)
    Meet Brood X of the 17-year Periodical Cicada.

    Snap! crackle! pop!

    Why waste such a monster on mere ants. I realize there are some of you out who would think of a certain movie and popcorn (lots of popcorn), but you've got to agree this is more unique.
    • I read an article the other day about a guy in Bloomington, IN that caught some cicadas, sauteed them in butter and garlic, and got hives.

      So don't eat 'em. But burn away!
    • but you've got to agree this is more unique

      Important note from the grammar police...

      The phrase "more unique" has recently been replaced by the more concise term "uniquer".

      Similarly, the phrase "most unique" is now "uniquest"

      Check out the difference between absolute and relative terms :-)

  • by Anonymous Coward on Friday May 21, 2004 @09:23PM (#9222105)
    This is just like the magnifying glass and ant game only this scales up to poodle sizes. Oh well. If I ever have a son...
  • Mindless (Score:3, Interesting)

    by Roland Piquepaille (780675) on Friday May 21, 2004 @09:24PM (#9222109)
    Chalk actually burns under this thing.

    Chalk burns eh? Creative chemistry, more like it. Here's another fun thing you can do: drop your "burnt" chalk in a glass half-full of water, let it bubble, and put your finger in it. Let me know how it feels.

    So do aluminum cans. They smell really bad.

    Aluminium doesn't smell bad when it burns. I suspect whatever soda pop chemicals remaining in the can do.

    It seems that normal concrete will start emitting plumes of smoke just before it pops

    As would burning tar, or any other heavy petroleum derivate.

    * Mike's car.

    Well, not yet. But it's plastic, so it would go up in no time at all. Or maybe we could just shrink-wrap the body around the frame.


    Try focusing the lens on the round plastic thing that smells funny, on the rear side of the car...

    Seriously, this article is all about playing with a new destructive toy and not much about using the toy in question to do interesting science-related experiments.
    • Re:Mindless (Score:3, Interesting)

      by Siva (6132)
      Here's another fun thing you can do: drop your "burnt" chalk in a glass half-full of water, let it bubble, and put your finger in it. Let me know how it feels.

      aw, at least explain for those of us who lack the chemistry knowledge and the stock of burnt chalk what will happen...
    • Re:Mindless (Score:5, Insightful)

      by DoraLives (622001) on Friday May 21, 2004 @09:39PM (#9222197)
      this article is all about playing with a new destructive toy and not much about using the toy in question to do interesting science-related experiments.

      Yeah, and I guess it shouldda been Smiley Captioned for the Humor Impared, too.

    • ...and? (Score:5, Insightful)

      by Sycraft-fu (314770) on Friday May 21, 2004 @09:40PM (#9222208)
      Playing with a toy is the POINT of a toy. I have a new Radeon 9800 Pro that, I am made to understand, has a pretty bitching vector unit that can be used for scientific calulations, rendering and the like. It is not used for any of these educational pursuits, however, and is instead used to render lightsabers which I then use to cut up bad guys it also renders. In other words, I bought it as a toy.

      It doesn't sound to me like they ever intended to do much science, it sounds like they intended to fuck around and burn shit, which they did with a high degree of success.
    • Re:Mindless (Score:3, Insightful)

      by Cecil (37810)
      Seriously, this article is all about playing with a new destructive toy and not much about using the toy in question to do interesting science-related experiments. ... you say that like it's a bad thing?
    • Re:Mindless (Score:5, Informative)

      by modecx (130548) on Friday May 21, 2004 @09:46PM (#9222245)
      1) Aluminum cans have a thin coating of plastic on the inside to provent the soda's acid from etching the aluminum. 2) they have all sorts of paint on the outside. That's what smells bad. Not left over soda.

      But, onto the other point: many geeks like high amounts of kinetic energy. It's true. Often, this love tuns into the irrational lust for wanton destruction of random objects. Sometimes, something is learned by the results, sometimes not. But it's the journey that's important (fun).
      • Re:Mindless (Score:4, Funny)

        by mlh1996 (446618) on Friday May 21, 2004 @09:58PM (#9222312)
        But, onto the other point: many geeks like high amounts of kinetic energy. It's true. Often, this love tuns into the irrational lust for wanton destruction of random objects.

        Which leads to my observation that, when presented with a large electromagnet, a REAL geek immediately constructs a railgun

        I mean, really. Is there anything more beautifully destructive than a railgun?

    • by SuperBanana (662181) on Friday May 21, 2004 @10:35PM (#9222506)
      It seems that normal concrete will start emitting plumes of smoke just before it pops
      As would burning tar, or any other heavy petroleum derivate.

      Concrete doesn't contain the slightest amount of petroleum. You're thinking of -asphalt-, which is entirely different.

      What smoked was contaminants on the surface of the concrete, and possibly some stabilizers. It popped because of the moisture in the concrete expanded- concrete doesn't handle much except external compression very well.

      Aluminium doesn't smell bad when it burns. I suspect whatever soda pop chemicals remaining in the can do.

      No, more likely the label ink.

    • Re:Mindless (Score:4, Insightful)

      by drinkypoo (153816) <martin.espinoza@gmail.com> on Friday May 21, 2004 @11:03PM (#9222659) Homepage Journal
      It seems that normal concrete will start emitting plumes of smoke just before it pops
      As would burning tar, or any other heavy petroleum derivate.

      concrete [reference.com]: "A hard, strong construction material consisting of sand, conglomerate gravel, pebbles, broken stone, or slag in a mortar or cement matrix."

      I do not think that word means what you think it means. It seems as if you are confusing concrete with asphalt [reference.com] or tarmac [reference.com](adam).

      Guess what, setting shit on fire is fun! If you are relatively responsible about it and don't light shit on fire accidentally and/or let things get out of hand (note: many forest fires grow from the cooking campfires of the incompetent) then really, who are you harming? Well, anyone breathing the vapors. But besides them?

  • just remembered we have one of these - 6' diameter - someone thought it would help with solar collectors back in the 80's - gotta dig this out!
  • I have one of these. (Score:5, Interesting)

    by Qender (318699) on Friday May 21, 2004 @09:24PM (#9222111) Homepage Journal
    I found one of these at my school last year. The first thing I did was take it to the parking lot to set paper on fire. The asphalt under the paper burned. I also melted pennies with it, and it can make holes in soda cans. Is there anything else anyone thinks I should burn with it? it's in my garage.
    • Is there anything else anyone thinks I should burn with it?

      Glass softens and flows not much hotter than the zinc melts. In the article pennies melted in seconds, so glass should be easy.

      I know enough about glass to warn you it might explode while heating and cooling. It can even "explode" after it's been cool for a while depending on the annealing. Try small peices over copper.

    • by ShaunC (203807) on Friday May 21, 2004 @11:30PM (#9222806)
      Is there anything else anyone thinks I should burn with it? it's in my garage.
      That depends. Do you happen to live near Lindon, Utah [sco.com]?
  • Laser Communications (Score:5, Interesting)

    by Rob Carr (780861) on Friday May 21, 2004 @09:24PM (#9222114) Homepage Journal
    Besides destorying things, these fresnel lenses cand be used for all sorts of constructive fun.

    A perfect example is a laser communication system. A laser beam can be modulated and used to transmit audio. The receiver needs to collect as many photons as possible from the laser transmitter - hence the use of the fresnel lense. Signals can be bounced off clouds - I've heard of transmissions going over 60 miles!

    The Amatuer Radio Laser Communications Page [qsl.net] has a good primer that has a link to a lot of the basics. And no, you don't need a ham license - although it helps!

  • Hanging wall art (Score:4, Interesting)

    by G4from128k (686170) on Friday May 21, 2004 @09:27PM (#9222134)
    Hanging a Fresnel Lens in front of a white wall projects a nicely focused image of the room onto the wall. Depending on the arrangement of the room and windowage, its poosible to watch the world pass by on projected image. The optimum distance from wall to lens is approximately the focal length (or a little farther if the subject is close to the lens.

    Just make sure the sun never gets to the lens or it will burn an arc across the wall.
  • Oh great (Score:2, Funny)

    by iminplaya (723125)
    Now the Dept. of Homeland Security is going to order various municipalities to block the sun [64.233.161.104].
  • by PissingInTheWind (573929) on Friday May 21, 2004 @09:28PM (#9222139)
    you'll love this flash game: Ant City [channel4.com]
  • Burning things has been done again and again and really isn't Slashdot (any news really) worthy.

    Unless you find a way to pull a MacGyver with it and foil some terrorist plot or something I can't say I'm too impressed.

    Ben
  • by NoMoreNicksLeft (516230) <john...oyler@@@comcast...net> on Friday May 21, 2004 @09:33PM (#9222158) Journal
    Could you do something serious with this? Put the damn thing in a rig that follows the sun, and a small steam turbine under it, just how much juice could it provide?

    I wish I knew the math to this, but damn, if it could provide even a small fraction of the power I use during the daytime... (by this, I mean 5-10%)

    Anyone want to impress me with their math/physics skills?
    • by nelsonal (549144) on Friday May 21, 2004 @09:46PM (#9222243) Journal
      If you follow the sun perfectly you could figure on about 1 kW of light (cheesy rule of thumb) hitting your turbine. Assume 30% efficiency and you have 300W of electric power generated for an average of 12 hrs per sunny day. Three light bulbs (or 6-8 florescents) or one fully loaded computer (300 W power supply, no electric heat, iron, or oven. Oh and you'll need a capacitor bank to handle your inductive load when motors start. Go grab your power bill and see how many kW/hrs you use in a month and then figure out how many fresnel lens/turbine array's you would need to achive that power. Storage would be a mofo, although you might be able to sell it into the grid in your area.
    • by Roland Piquepaille (780675) on Friday May 21, 2004 @09:48PM (#9222253)
      I wish I knew the math to this, but damn, if it could provide even a small fraction of the power I use during the daytime... (by this, I mean 5-10%)

      - In bright summer daylight, at noon the sun provides 1200W/m2

      - This fresnel lens is 80x100, so captures 1200 * 0.8 = 960W at best

      - A good steam engine, with a condenser and exhaust reheater provides has an efficiency of about 30%, so it would give 960 * 0.3 = 288W in mechanical power

      - A good alternator, going at its preferred RPM (not necessarily that of the steam engine's prefered RPM, but let's assume) has an efficiency of about 90%, so it would give 288 * 0.9 = 260W

      So you'd get 260W in the best possible conditions, in the brightest of days, in summer, at midday. Throw some clouds and, assuming the entire thing doesn't stall and stays at its nominal efficiency (not likely, but let's assume), you get about 6 times less power, so about 43W

      In short, you're better off with solar panels: perhaps a little less efficiency for the same price, but more surface and a lot less aggravation.

      • Can't you use fresnel along with photvoltaic to improve the power of the photovoltaics? Maybe not this size of fresnel, but smaller, cheaper ones to get a boost on your cells.

        I could be wrong...
        • Can't you use fresnel along with photvoltaic to improve the power of the photovoltaics? Maybe not this size of fresnel, but smaller, cheaper ones to get a boost on your cells.

          If you pair a photovoltaic panel to the same size fresnel lens, no because you'll just have concentrated the same amount of light on a tiny bit of the photovoltaic panel instead of having the same power spread on the entire surface. You can however increase the power to a smaller panel, because then you concentrate on this smaller su
    • I have given much thought to this, and in fact have done the math. Though it is a hassle to repeat here.

      I came up with a general number -- 1.5 kilowatts, this is about the amount of energy you would need to propel a small car (like a volkswagon beetle, old variety). This is about the same as a small gasoline engine could provide.

      While it looks like a lens like this might be able to provide the power, there are a number of issues that need to be addressed.

      One is continual focus of the lens. The position o
  • by Fiz Ocelot (642698) <{baelzharon} {at} {gmail.com}> on Friday May 21, 2004 @09:40PM (#9222207)
    Would it be possible to build a giant one, say 100 square feet, and focus it into a fiber optic type of cable, essentially using it for a cutting laser?

    Or what if you had a few massive ones in space, could you focus enough energy for use in a fusion reactor? You'd amplify all of that free energy and I don't really know what I'm talking about. Perhaps you could attach it to a shark's head.

  • by cybermace5 (446439) <g.ryan@macetech.com> on Friday May 21, 2004 @09:41PM (#9222213) Homepage Journal
    Not as big, maybe 14 inches across, but overhead transparency projectors have a big square fresnel lens in the base. Since a lot of businesses, schools etc have moved to LCD projectors, you might be able to find an old overhead that no one cares about. Still concentrates a lot of light; you can't look at the spot and it'll burn lots of things. Probably not metal, though.

    Cooking idea: Take a length of thin all-thread and turn it with a slow motor, with a matching nut fastened to a board so that the all-thread and motor are slowly pulled along. Spear a few hot dogs on the all-thread and set the lens to a medium concentration. Spin up the motor, and the sun will cook the hot dogs in a spiral....
  • by Danny Dale the Not-S (778706) on Friday May 21, 2004 @09:52PM (#9222279)
    of a Nuclear Bomb.

    Imagine kicking over an ant hill, then frying thousands of the little fuckers with each sweep of the beam when they come pouring out. Considering you can melt nickels and cut soda cans in half with this thing, it's possible you can actually [i]glass[/i] that anthill!

    Just make sure it's fire ants. Those bastards deserve it...
    • You could always use the and neutron bomb-boiling water. My mother used to do this to get rid of ants nests and it was a little frightening. Basically a kettle of boiling water kills all the ants instantly.
  • by Flower (31351) on Friday May 21, 2004 @09:52PM (#9222281) Homepage
    Do not taunt giant Fresnel lens.
  • Ah, jr. high science fair. Back in '72, Edmunds' lens was only about 1.5' square. I found a design for a 4' tall wood test rig at the library, which wisely suggested a kiln brick at the focal point. I chiseled out a small pit to hold samples, and went to work.

    It made a great project, the most sophisticated object I had built up to that point. It blew as a science experiment, since I didn't have a plan of action other than to melt things, nor a thermometer that could measure it's limits. In retrospect a turkey probe might have worked. I did succeed in liquifying a number of types of solder.

    I only rated a participation ribbon at the fair, but one of the science teachers took it off of my hands for $75, recouping my (dad's) material expenses and then some.

  • by tokachu(k) (780007) on Friday May 21, 2004 @10:02PM (#9222328) Journal
    You can get a free Fresnel lens by doing a bit of dumpster diving. If anyone has thrown out a 50" projection TV, the lens is yours!

    NOTE: This HAS happened; I am NOT being sarcastic. I took the Fresnel lens out from the trash and stuck it under my bed, wondering what I could do with it. Now I know! (perhaps I should just eBay it for $100)
  • by Cirrocco (466158) on Friday May 21, 2004 @10:05PM (#9222340) Homepage
    If this thing is capable of creating such intense heat (with, as far as I can tell, very little environmental impact such as that created when making solar panels) then perhaps it could be used as an alternative (and portable) power source?

    I need to look into this. Heat energy can be converted into electric energy, even if it isn't all that efficient.
  • Projection tv's use them!
    since I repair them for a living, ive actually done this.
    its quite fun, but do be careful!
    I like using the fresnel from a 60" projection tv the most :)
    I have burned up phone books in no time with it, and I have tried cans, I got one to melt.
    next time you see a projection tv in the trash, get the lens.
    the lens will be the innermost of the 2 (or 3, if there is a protective screen)
    have fun!
  • ... and I come to the $99 part. The first thing she says is, "You can't have one."

    "But a professor," I try to explain...

    "You can't have one."

  • by Cirrocco (466158) on Friday May 21, 2004 @10:27PM (#9222452) Homepage
    I can't believe, with all the talk of putting one in space, nobody has used the words "DEATH" and "STAR" just yet.

    Glad I'm the first. I think.
  • by Zirtix (443841) on Friday May 21, 2004 @10:30PM (#9222468) Homepage
    I read a paper once that advocated the following strategy for getting to Proxima Centauri in a span of ~50 years. The plan is this:

    1) Construct array of solar panels near Mercury (or whatever)
    2) Beam resulting gigawatts of power to the Moon using small lasers/masers
    3) Collect the power and use it to feed a very large laser
    4) Point laser at a huge fresnel lens orbiting Jupiter (say)
    5) Point fresnel lens at a solar sail, accelerating it to ~0.1c quite quickly

    The lens allows your laser beam to stay focused at long range (like 4 light years). Of course it would take centuries to build the kit needed, but once it's running you can send lots of payloads for little cost (solar sails are 'cheap' to make). There are also solar sail strategies for interstellar return journeys!

    I like solar sails, generally. Sustainable space travel!
  • by lightray (215185) <tobin@splorg.org> on Friday May 21, 2004 @10:33PM (#9222487) Homepage
    You can also build your own giant spiral fresnel reflector [splorg.org] at home.
  • by earthforce_1 (454968) <earthforce_1@yaho o . com> on Friday May 21, 2004 @10:35PM (#9222504) Journal
    Behind the Strategic Defence Initiative missile shield. A giant magnifying glass in space, to burn incoming missiles, or enemy cities.
  • pfft (Score:4, Funny)

    by ShadowRage (678728) on Friday May 21, 2004 @10:46PM (#9222569) Homepage Journal
    I melted asphalt with good 'ol gasoline.
  • Watch Your Eyes (Score:5, Informative)

    by OceanWave (192467) on Friday May 21, 2004 @10:53PM (#9222598)

    One thing that cannot be overstated is the use of eye protection. And, whatever is selected for that application must handle IR as well as visible light. (Nearly all of the UV is absorbed by the plastic the lens is made out of, so it is not much of a factor.)

    Using such a lens, to focus solar radiation, can produce power densities equivalent of a Class-IV laser; where the warnings typically read "avoid exposure to direct or scattered radiation". Even if focused to a spot size of 4cm^2--at an estimated 1kW--the power density would still fit 2.5W/mm^2. This is the same level as a 10W laser, with 2mm beam focus.

    Granted that the focus is only at one point, it is easy to overlook when scattered radiation--from a "point" source--can be dangerous.

    As the article states, use very heavy welding goggles, and maybe have some sunglasses on under those! It is also recommended to ensure that the goggles cover the infrared parts of the spectrum effectively.

    Also note: laser safety goggles would be ineffective for this application, due to the fact that they typically use dichros, which typically are not very "wide-band". They reflect very specific wavelengths--very efficiently. But, since solar radiation is very wide-band, a lot of it will still get through.

  • by Dirk Pitt (90561) on Friday May 21, 2004 @10:54PM (#9222602) Homepage
    ...the original uber-cool use of the Fresnel lens, namely, in the first actually useful [lighthouseratings.com] lighthouses.
  • by rspress (623984) on Friday May 21, 2004 @11:00PM (#9222636) Homepage
    Find the biggest "beauty mirror" you can. These things have a regular mirror on one side and a 5X mirror on the other. Use the 5X side to focus a beam of destruction wherever you wish.

  • Childhood Anecdote (Score:3, Interesting)

    by donscarletti (569232) on Friday May 21, 2004 @11:43PM (#9222884)
    Back in primary school we were given Fresnel lens of about half that size (still obscenely huge) as souvenirs by a short sighted tour guide. Needless to say they were confiscated immediately by the teachers so they would not be used during the course of the excursion. The problem is that they were redistributed on our first day back home, back in a sleepy drought riddled town in western New South Wales, Australia.

    Lucky the fire on the oval was able to be contained, otherwise I would have lost more than my Fresnel lens.

  • by cybersk4nk (727689) on Friday May 21, 2004 @11:49PM (#9222918)
    this article is interesting because it reminds me of something i read in an old history book on mayan temples. apparently, some of them have rock cut so exactly and perfectly, a knife blade will not fit between the seams. i remember reading (or watching on tv once) about how some scientists/archeologists theorized that the maya used focused sunlight to cut the rock (specifically, big gold encrusted sun discs), and how preposterous others thought of the idea. i even remember that some scientists tried it out once with gold polished mirrors, and it failed utterly. now that we know a giant fresnel lens can burn ashphalt and make concrete crack and pop, i wonder if the maya came up with a similar technique based on a more primitive (or more advanced) fresnel-like lens. anybody want to carve up some rock to test the theory? it would make for some fun mad science to prove an old theory.
  • Archimedes (Score:4, Interesting)

    by vlad_petric (94134) on Friday May 21, 2004 @11:55PM (#9222942) Homepage
    Legends say that Archimedes [ucar.edu] wreaked havoc through the Roman invasion fleet trying to conquer Syracuse with giant lenses.

    While it's very hard to verify this legend, one thing we know for sure is that Syracuse was conquered via land, and Archimedes ingenuity had an important part to play in defending Syracuse from the sea.

    So yeah, this is stuff that matters, but hardly "news"

    • Re:Archimedes (Score:3, Insightful)

      by WillWare (11935)
      The I heard it, he didn't use lenses, he used mirrors. He got the army to issue polished metal shields to all the soldiers, that could be used to cast shield-sized reflections onto the sails of approaching ships. If all the soldiers put their reflections on one spot on the sail, it would set the sail on fire.

      I think you're right that this story is probably apocryphal. It's hard to imagine that good mirrors would have been inexpensive enough in those days to make this practical. It's also difficult to imagi

  • by Anonymous Coward on Saturday May 22, 2004 @01:27AM (#9223311)
    I recall reading an article in a Canadian electronics magazine back in the mid 80s where the author created a satellite "dish" based on Fresnel theory. It wasn't a dish at all, but a large plywood Fresnel lens that focused the (C-band) satellite signal onto a feed horn behind the plywood (as opposed to a dish where the feed horn is located in front at the focal point). I don't remember if the plywood was painted with a metallic paint.

    I think the mag was Electronics Today and the author may have been Steve Rimmer or David Stringer. Those guys used to do all kinds of crazy things, like mounting a dozen larger speakers (covered with sheet metal) to the front of a VW van and hooking them up to a frequency generator and amplifier. They used this rig to distort the bounced signal from a police radar gun tricking it into displaying a speed of their choice
  • by Doc Ruby (173196) on Saturday May 22, 2004 @01:01PM (#9225285) Homepage Journal
    This lens, or the current replacement model (35" radius) at $225, seems cheaper than a m^2 solar cell. How about mounting the lens over a small cell? Where can I find the efficiency curves for solar cells, graphed across the total incident light wattage? If the curve peaks above the ~400W:m^2 incident on my roof at noon, I can mount the cell to intersect the cone of focused light, along the way to the focal point. Judging from the experimental results at the "Fresnel Destruction" site, the focal point itself probably offers "nonlinear" power transfer (exploding cell). But somewhere in between might be a cheaper solar collection array. OTOH, if cells' max transfer efficiency is at below 400W:m^2, maybe it's time to consider this concentrator on a glass/water->steam/turbine. In which case, where are the efficiency curves for that apparatus?

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