Random Destructive Acts via Focused Solar Radiation

Updated 2003.03.19
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Random Destructive Acts via Focused Solar Radiation

When I was a kid, I always wanted Edmund Scientific's Giant Fresnel Lens. "Melts asphalt in seconds!" the ad said. When I went to graduate school I 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.

Unfortunately since I am now in California and the beloved Dershem Memorial Lens is still in Michigan, as is most of the, er, evidence, I can not at this time provide any images.

Things we have destroyed with the giant Fresnel lens:

As you can see, the purchase of such a dangerous item is mostly a self-destructive act. Be warned.

There are probably many more otherwise useful and productive things which have met an untimely demise through our devious misuse of science. The memories are hazy (probably because of the fumes from all the burning plastic) but as they come back I will add them here.

The Random Destructive Acts FAQ

Updated March 19, 2003:

It has been about 8 years since I wrote this page (before 2002 the last modification date was June 30, 1995) and I still get emails about it every few days. The most frequently asked question is, "Where can I buy one?" Edmund Scientific quit making this rectangular lens a few years ago and the company split, but Edmund Industrial Optics now makes a round 35 inch diameter lens (and many smaller sizes). Last I checked it was on this page but that's likely to change. It was part number NT43-921: FRESNEL LENS 35.0" DIA and cost $224.10.

Jacob Dickinson informed me that Alltronics.com sells a rectangular lens similar in size to the infamous lens we had, and a few weeks ago I got one (see below). It's thinner than the Edmund's lens we used to have and needed a frame, but it's big and about half the price of Edmund's lens. You can find it at http://www.alltronics.com/lenses.htm and it was $99 plus shipping when I looked.

If you're looking for something smaller and a lot cheaper, you can often find cheap surplus (used) 12" Fresnel lenses at American Science and Surplus and Scientifics (the new owner of the Edmund Scientific Catalog) for as little as $3.

And to answer another only slightly less frequently asked question, no the beloved Dershem Memorial Lens is not for sale. We decided it would go to the first of us to get a Ph.D. (in other words, to first order we left it up to fate) and Chris Chaput was the proud winner. Thus Dr. Chris now owns the lens and (for obvious reasons) it is kept at least 1000 miles away from him at all times.

People also ask, "What was the area of the focus?" The best we ever got was about a centimeter across. The lens was somewhat flexible so the corners didn't tend to contribute much. A professor at U of M who had one of these said he built a frame with cross supports and was able to get more of the lens to contribute to the focus.

A lot of people want to know what temperatures we achieved. It's hard to make this estimate, the real question should be how much power the lens can supply (and you can see more about this below). But in terms of estimates, we certainly melted aluminum which has a melting point of 660.37 degC. We never quite managed to melt copper with the old lens but (melting point of 1083.4 degC) but we have with the new one (see below) so I think we could have if the sidewalk had not been so fragile. Assuming a black body target and that the sun supplies 1340 W/m^2 (before atmospheric absorption) we calculated that we should be able to melt quartz (1610 degC) but in practice I don't believe we achieved temperatures that high.

The New Lens

Here are a few pictures of our new lens (from Alltronics.com) and our first test target.

The target was (certainly past tense) a zinc penny. The copper turned into a black crust and the zinc ran out the side (that silver blob going up in these pictures). The target stage was some shale like rock we had sitting around, which would flake when heated but didn't pop and eject the target.

We have since taken many targets and caused them to melt/incinerate/explode (it's a pain to clean pop off the lens) and soon I will add a few more pictures.

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:

This is likely at least a slight underestimate of the lens' output for at least a couple reasons: it was slightly cloudy and the sun was not all that high (effective transmission may have been much lower); likely a lot of the energy went into the stage instead of the target (the metal stage we used later was pretty badly warped when we were done). We hope to improve on this by making stands and will try to measure it more carefully. This is optically a better lens than the old Edmunds one (better image) and as Edmunds points out, optically better Fresnel lenses tend to be less efficient light concentrators, so maybe that's part of our problem too -- there was a nice central focus (smaller than the penny) but a lot of the light was scattered into a much larger area, at least a foot across. This may make the lens more useful for diffuse heat applications (cooking) than concentrated ones. We'll see . . .

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