Monday, March 23, 2009

Light and Heat: Giant Fresnel

We all played with magnifying glasses as kids. Lighting leaves on fire, scorching our retinas, and causing a massive downswing in the local insect population. Fond, fond memories.

If you didn’t, may I suggest that you go find one, fool around with it on a sunny day for an hour or so, and then rejoin us. You are missing out.

A few kids were lucky enough to join the big leagues with their big Fresnel lenses. I was one of those much-envied kids. I cannot imagine how a twelve inch square of plastic could bring more joy to anyone. It was instant fire, molten lead, and ant vaporizing. At that scale it ceased to be cruel… the ants simply ceased to be.

Flash forward nearly two decades. I’m hard to impress (though easy to entertain). Fantasticked out by thermite and plasma torches, how could a little magnifying glass impress me? When it’s a very big one. Three feet by four feet. The size of a 60″ TV (funny coincidence there…), this is NOT your childhood playmate.

Welding goggles are a necessity here, not just a madscipunk accessory. Dead men don’t tell tales, and blind men have trouble negotiating traffic.

250 watts/m^2 of insolation is a pretty fair estimate. 12 ft^2 = 1.1 m^2, so nearly 275W of solar fire juice hitting the lens. Part of that total is spectrum that won’t pass through the lens, plus lens losses on the visible band, plus dirt… so an ultraconservative estimate is around 200W coming out the other side. This is concentrated into an area the size of a penny. A very sad and runny penny.

200 watts may not sound like much, but imagine a laser of the same power. The most powerful of the Wicked Lasers caps out at 300 milliwatts… 1000x less powerful. Laser cutters are generally 100W and less. This is serious power.

How does that translate to real live burnination? Melts pennies, brass locks, glass, and even fuses sand. Fusing silica = 1500°C. I’ve even kicked around the idea of a solar fired kiln.

Now that you’re frantically searching craigslist and ebay for big Fresnel lenses, I’m going to let you in on the secret. Rear projection TVs all have lenses equal in size to their display. It’s part of the core design. These are the same TVs that people are throwing away at this very moment, all over the US. The Free section of Craigslist in LA sees at least three of these bad boys a week, often more. Tax rebates, the Superbowl, and Christmas all cause peaks in the dump rate.

Be considerate - if the TV is in working condition, let someone else enjoy it. Also, you’ll probably be expected to dispose of all the RPTV materials if you pick one up, so don’t strip out the screen and leave the heavy CRT-laden base on your benefactor’s curb.

The last tip is to build a frame. This supports the Fresnel and prevents sagging. If the lens sags, it distorts the focus, and prevents you from achieving the highest possible concentrations. I also added a piece of aluminum bar stockto further reduce sag in the center. It blocks a tiny percentage of the light, and really improves the focus.


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