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Andy starts building a Green Power-Ray* *it was originally called a "Death-Ray" but the name was changed to confuse Homeland Security Spring has sprung and a young man's mind turns to fancy. Since I am now officially old, my mind has turned to "How can I play with a big-assed magnifying glass and burn things while not getting in trouble with the authorities?" The answer is, of course, "Get people to think I am doing it to get kids interested in science." I was inspired by some videos on YouTube to try and find a way to reenact the Siege of Syracuse, only I want to find a way to make this into a participatory event, where I'm using a largish Fresnel lens to concentrate sunlight. How it'll actually work (or not) will be the subject of continuing write-ups. This is the first: Step 1: Getting a largish Fresnel Lens on the cheap. You can buy Fresnel lenses, but they are also key components in old-style rear-projection TVs. Remember these beasties? Giant cabinets for low-def TV and you had to pretty much sit dead-on centered to get the best picture? They cost thousands back at the turn of the century. Now, with a little patience, a person can pick one up for free, off Craigslist.
This is the one I got - a 47" Mitsubishi. Picked up in Bethel from a very nice family. It was a heavy bugger, so I had to borrow a trailer, but there it is.
The front came off pretty easily . . .
. . . revealing three 4" bonus lenses inside. I'll figure out what to do with them, later.
They come apart really easily, They have one really thick lens, the others aren't very impressive.
There's also a pretty cool mirror in there. I wonder if I can use it for targeting?
I started taking the rest of the unit apart and discovered the individual projectors have a cooling liquid inside. Huh, go figure.
The gold is in the front screen. There are three screens, actually. A 3mm thick plastic front screen, probably more for protection than anything, then the Fresnel lens, and a collimator lens.
There might be a reason you can buy Fresnel lenses for $300 and up. The ones you get out of TVs are opaque and really bendy. Bendy will cause problems with focusing.
This is one of the problems with solar-powered death-rays. Testing was delayed by a day.
I rushed home from work one late afternoon - the sun was setting behind the trees behind my house. I only had a few minutes to do some testing. The Fresnel lens is taped so it is resting on the thicker TV screen, hopefully laying as flat as possible. My target was a candy thermometer on a board. My goal today was twofold: Figure out the focusing distance for the lens and get an idea of how badly performance will degrade if I cut the lens into a round shape instead of rectangular (I think it'd look cooler as a round lens.)
The internet didn't tell me how much of the power of the lens comes from the the edges, so I wanted to see what happened if just the center was active. Turns out: Output degrades a lot, about 1/3rd if you cover about .1/3rd of the lens.
Even completely uncovered, the performance was less than stellar. It got up to about 250° and just slightly scorched the wood. I was unable to focus it any tighter than a 2" dia dot - a wiggly 2" dot because I was holding it up manually. Issues to overcome: I need to get the lens as stiff as possible - I'll put it in a frame for that. I need to put it on a stand, too, to hold it steady. |
