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Things You Can Do With A Giant Fresnel Lens 469

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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  • by Anonymous Coward on Friday May 21, 2004 @09:40PM (#9222200)
    You can still access it by copy/pasting the link.
  • by cybermace5 ( 446439 ) <> 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....
  • 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).
  • 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.
  • Re:Mindless (Score:5, Informative)

    by dgatwood ( 11270 ) on Friday May 21, 2004 @09:49PM (#9222266) Homepage Journal
    Chalk is limestone. If you burn it, you get lime (calcium oxide). Add water, and you get calcium hydroxide. It's a very strong antacid.

  • Re:wtf (Score:4, Informative)

    by timmi ( 769795 ) on Friday May 21, 2004 @09:51PM (#9222276)
    It's pronounced fer-NEL and it's spelled Fresnel because it's named after the french guy who invented it.
  • Re:Mindless (Score:5, Informative)

    by Lehk228 ( 705449 ) on Friday May 21, 2004 @09:51PM (#9222277) Journal
    antacid= Base, which is worse for skin than acid, Acid you will feel and pull your hand away because the human body can feel the burn from acid, a base will eat through your skin and you will just feel "slippery" s it turns your flesh into soap
  • Re:Mindless (Score:2, Informative)

    by mlh1996 ( 446618 ) on Friday May 21, 2004 @09:53PM (#9222282)

    Ok, working from memory here, and being lazy, so I'm not going to bother balancing equations:

    Chalk (Calcium carbonate) + Oxygen -> Calcium oxide + Carbon dioxide
    Calcium oxide + Water -> Calcium hydroxide

    Calcium hydroxide is a strong base, so putting your finger in it will probably result in severe alkali burns.

  • Re:Scaled Up (Score:2, Informative)

    by jd_esguerra ( 582336 ) on Friday May 21, 2004 @09:53PM (#9222288)
    IANAPH but it the lens was say 300 feet by 300ft, would it still be possible to focus the light to a 1cm point? And if so, does the size of the lens increse the temperature?

    Probably not, and yes. A lens of that size would be extremely difficult to build and operate. Instead, you'd use an array of smaller lenses, or even better, mirrors. If you look at large telescopes, most use mirrors in part because of weight issues and manufacturing issues with lenses over say 1-meter.

    The size of the lens does increase temperature, or at least the energy density at the focal point. A bigger lens (or mirror) can capture and direct more solar energy.

  • Not really (Score:2, Informative)

    by chamblah ( 774997 ) on Friday May 21, 2004 @09:56PM (#9222308) Homepage
    Aluminium doesn't smell bad when it burns. I suspect whatever soda pop chemicals remaining in the can do.

    And very possibly the paint they use to put the logo etc... on the can. He also stated that the aluminium can smelled really bad, not the aluminium that the can was made of. So when referencing to the can in that way would mean everything involved that makes it an aluminium can.

    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.

    While the expierments they did were fun, then did put some science into it.

    The FAQ []

    Impressive as destroying a penny may seem, I estimated that we may have only managed to get maybe 10 percent of the available energy hitting the lens (roughly 1kW) into the penny:

    * Mass of a zinc penny: 0.0025 kg
    * Specific heat of zinc: 390 J/kgK
    * Melting point of zinc: 419.58 degC
    o Thus 20degC to about 420degC takes 390 J
    * Latent heat of fusion for zinc: 1.1x10e5 J/kg
    o Thus to melt the penny takes about 275 J
    * We heated the liquid zinc considerably as well, but I will ignore that.
    o Total energy in the penny: > 665 J
    * It takes something under or around 6 seconds to melt a penny:
    o 665 J / 6 sec gives us a lower limit of about >= 111 Watts
    * Sunlight at the earth: 1365 W/m^2
    * Transmission of the earth's atmosphere: maybe 65-70% at this angle with some clouds?
    * Area of the lens is about 1.1m^2
    o Power on the lens: approx. 1000 W
    o Power to the penny >= 10.0%

  • by mikeophile ( 647318 ) on Friday May 21, 2004 @09:58PM (#9222311)
    Something like this? []
  • Re:Ideas (Score:5, Informative)

    by edheler ( 715806 ) on Friday May 21, 2004 @10:14PM (#9222381)
    Here [] are a few [] references [] to increasing solar cell output with Fresnel lenses. Enjoy!
  • 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!
  • by lightray ( 215185 ) <> on Friday May 21, 2004 @10:33PM (#9222487) Homepage
    You can also build your own giant spiral fresnel reflector [] at home.
  • 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.

  • It's being done! (Score:5, Informative)

    by MarcQuadra ( 129430 ) * on Friday May 21, 2004 @10:38PM (#9222521)
    that's actually how some solar power stations work. They have a bunch of mirrors that aim the sunlight to a glass globe filled with water, the water boils out and powers a turbine, condenses out and returns to the globe via a valve and pump.

    Overall it works better than solar cells because it's so simple and you harness the heat energy rather than the light itself, but therer's only economy to it on a large scale, you need enough space to get a huge amount of water to constantly boil. Also, it's significantly harder to get this thing working on less-than-ideal days; solar cells still collect juice on slightly cloudy or overcast days, but this method doesn't work nearly as well.

    Still, a good way to apply solar energy when in conditions that permit. I'd like to set up a small unit with a fresnel lens and 'boiling globe' to generate hot water (which I'll pump through a radiator) for my house in the winter. The problem I see is with safety, that beam has to be EXACTLY where I want it or I'll burn the house down.
  • 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 [] 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.

  • by Rob Carr ( 780861 ) on Friday May 21, 2004 @11:18PM (#9222748) Homepage Journal long as you keep a careful eye on the clouds, because if they suddenly disperse, you'll be focussing daylight on your receiver...

    To keep down the S/N ratio, most long-range laser comm experiments are done at night. I suppose safety is another good reason for working after the sun goes down!

  • Re:It's being done! (Score:5, Informative)

    by Rich0 ( 548339 ) on Friday May 21, 2004 @11:38PM (#9222855) Homepage
    Sounds like it has a lot of advantages - like simplicity. The turbine of course is full of stuff that can break, but the rest of the whole operation has no moving parts, which is a big plus. Scaling up has to be fairly easy as well - just add a turbine and extend the size of your glass farm.

    Regular solar arrays need to have panels or mirrors that track the sun - lots of stuff to break over a large area, fancy focusing algorithms and sensors and motors...

    Often simple is best with stuff like this - that's how many home solar heating systems work - forget completely replacing the grid, just pipe some water through panels on the roof to heat it up...
  • by Anonymous Coward on Saturday May 22, 2004 @12:32AM (#9223071)
    Thermocouples are solid state and will generate electricity from differences in temperature. A company called Hi-Z produces some nice looking modules. Some interesting applications and specs are available at their site. []
  • Re:Watch Your Eyes (Score:4, Informative)

    by cmowire ( 254489 ) on Saturday May 22, 2004 @12:37AM (#9223090) Homepage
    Generally, you want either a gold-coated welding lens or an extra gold UV shielding glass to view this sort of phenomena... tm l
  • by Markarian421 ( 781998 ) on Saturday May 22, 2004 @12:44AM (#9223123)
    Try American Science and Surplus (, much more amusing catalog, and way cheaper than Edmunds.
  • Re: Melting Glass (Score:1, Informative)

    by Anonymous Coward on Saturday May 22, 2004 @12:52AM (#9223146)
    No, it's easy, providied you're not dumb enough to just put a piece of glass on the ground and shine the beam at it. Like most hot, melty things, molten glass is best heated in a crucible. Crucibles, surprisingly enough, are opaque and usually the right color to absorb heat - that is, they are blackened.
  • Re:Burnination (Score:2, Informative)

    by WalksOnDirt ( 704461 ) on Saturday May 22, 2004 @02:04AM (#9223445)
    I bought an 11" square fresnel lens (from Edmund's) when I was about twelve. The most refractory material I was able to melt was filings from a nickel coin, which were (and I suppose still are) mostly copper.

    The temperature you can achieve is mainly determined by the ratio of the diameter of the lens to the focal length. A larger lens with the same ratio lets you heat up larger objects, but it won't get small objects much hotter. A penny was much too large for my lens to have a visible effect on.
  • 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?

Forty two.