Re: increasing fresnel lens light concentration?

Others have offered good optical advice. I just wanted to point out that
no arrangement of lenses and mirrors (i.e., passive concentrators) can
produce a temperature higher than the effective black body temperature
of the surface of the sun (around 5250degC), or even reach that, since
there are atmospheric losses. And since you are only surrounding one
side of your target with the sun, I think that means you are limited to
something more like 4370degC. In reality, of course, coming anywhere
near these temperatures is an accomplishment.

With or without lampblack on the target, he melted zinc (topping maybe 300 degrees celcius, or roundly 600K; sometimes enough to cook solid waste) but managed to neither vaporize it nor ignite it. And whatever he used to determine the melting has not ruined an ability to e-mail or his lens. Contrawise coming within miles of a sliver of the surface of the sun would tend to age equipment prematurely, the way it throws off UV, heat, miniature 8000K solar-scale magnetic plumes and particle jets.... Clearly if the move to 60" flatscreen magnifier-viewers comes to involve optics which are strongly (5000:1) birefringent over a wide spectrum though, that ratio of concentration becomes more accessible than it was with mere terawatt lasers, SHG/THG, 40T magnetic confinement under high vacuum, and subtlety with the plasma state.

Big 32" I.D. shutters are pretty cool in themselves. Cristo wraps a solar kiln, anyone?

Sure the focus is still diffused over about 4x the ridge height, but that is pretty tight. Fixing the achromacity of the lens by finding the IR focus (or focus ring) with a perforated shutter and flash paper, and a ring reflector nearby to tighten it up to the optical focus, may not be worth the trouble and insertion loss.

Try to suspend your target in mid-air somehow (tungsten wires, maybe?),
so there is less opportunity to conduct heat away. Someone suggested
blackening it to increase absorption; do that but leave it shiny on the
back side to minimize re-radiation.

Lampblack is neat stuff, and runs to about 2000K before oxidizing (but see the MSDS.)
About 4 layers of that and you are talking about an oven better than a low-alumina-fiber-brick-lined pit; though metal shells as you describe are perhaps easier to maintain, since that is how production solar ovens go. (They also cool off rather than present a nighttime hazard.)

Perhaps a column which fixes atmospheric carbon from C02 can be powered, for use where plants are unwanted.

oh, sunlight-visible pdf readers are only getting farther off this way.
.

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