Giant 4ft fresnel lens, $50

[wbeaty]

Here's something interesting I just saw in a sales flyer:

  Giant 31" x 40" Flexible Fresnel Lens - G13994

 
  http://sales.goldmine-elec.com/prodinfo.asp?prodid=9039

DOesn't give the focal length though.  I saw someone at the Seattle UW street fair
using something similar to make jewelery; melting various colored glass
rods with sunlight to make "raindrop" shaped earrings.

DON'T leave such a large lens indoors without covering it.  If a sunbeam
should hit it wrong, even the reflection from the grooves can start a
fire.

Something that does work:  solar-powered wood burner.  Get a metal-jacketed
fiber optic bundle (glass fibers, not plastic, sometimes these show up in
surplus catalogs.)   Stick one end at the focus, use the other end to
write your initials on a 2x4.   But the end of the light-pipe at the focus
gets cookin.   Probably needs a metal block with water cooling if you want
to do it right.

The same catalog also has 400nM UV LEDs for $0.99.  For Burning Man, cover
your body with tattoos of tavern-rubberstamp invisible ink, then wear an
ultraviolet tiara?  (Or buy those expensive UV-fluorescing contact lenses,
for Blinking Alien Eyes effect.)

   http://sales.goldmine-elec.com/prodinfo.asp?prodid=7291

[Nando]

Well I have one of those, that I bought during Hamcom in Arlington Texas this past June for 5 dollars.

These Fresnel lenses are used in projection TVs.

At the moment I do not remember the focal point but one can calculate the energy if you assume 750 watts per square meter concentrated to 10 mm^2 to 100 mm^2 ( 10 to 100 square millimeters )

In July summer, noon time, I melted 3 onces of solder in about 30 or 40 seconds.

Regards

Nando