low-cost LED-based "one sun" light source

Winfield Hill · 2016-11-01T08:08:12+00:00

Does anyone here know about making a low-cost LED-based "one sun" light source? Commercial sources are large and expensive. Yes, they have very accurate spectral characteristics, and are uniform, repeatable and calibrated, etc., etc. But hey, what about making a simple LED light source of about the right intensity, maybe crudely replicating the dominate 400 to 750nm region? Or maybe just forget that aspect, and concentrate on a few useful wavelengths? -- Thanks, - Win

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Jamie M 9 years ago

Hi,

I don't disagree about the main principle that the sun's light can't be focused more than a certain magnification given by the distance from the sun and how big it appears in the sky. ie according to this article:

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theoretical maximum concentration ratio for a conventional solar concentrator with optimal optics turns out to be about 46,000. As the viewing angle of the light source gets larger, the theoretical maximum

other words, a uniformly cloudy day), the theoretical maximum concentration ratio is only 1.""

So if the sunlight at earth was 1kW/m^2, it could be concentrated to a theoretical maximum of 46,000 kW/m^2 by a solar concentrator, presumably this would remain below ~5800K (sun surface temp) when used to heat a conventional black body. I guess a question would be, does this magnification relate to the surface heat given off at the sun's surface?

Given the sun has an approx surface area of 6.09 x 10^18 m^2, and outputs something around 3.6 x 10^26 watts, that is about 59113 kW/m^2 at the sun's surface. That is pretty close to the 46,000 kW/m^2 above, and probably is greater since I used 1kW/m^2, when actually that value can be higher at the surface of the earth. So I will assume those numbers match, meaning it is theoretically possible with perfect optics to nearly match the surface heat of the sun, but not exceed it.

about using use multiple light paths to increase the 46,000 kW/m^2 onto a common target? I guess the question would be, if you add more light

decrease the max allowable concentration ratio, to effectively maintain the max of 46,000 kW/m^2, seems a bit weird that you can't just beam some more focused light from a different angle and surpass the

59113 kW/m^2 at some point and thus get above the threshold to create a black body on earth with temperature greater than 5800K.

A more complex solar concentrator target could use near field or other effects to potentially increase the heat beyond the 46,000 kW/m^2 by creating hot spots and cold spots in different areas of the target black body, or some other type of "heat engine" effect maybe, or a plasmonic particle accelerator could be powered by the sun with the beam focused onto a smaller black body.

cheers, Jamie

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Chris Jones 9 years ago

Haha. What if a single gamma-ray photon hits it, then it will instantly exceed the temperature of the sun and explode and kill everybody!

Or, maybe the number of photons matters too.

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Jamie M 9 years ago

Hi,

That would be an example of a passive system that exceeds the surface temperature of the thermal source, for a finite time in a finite area, which I think is valid (for x-ray and UV components of a 5800K thermal source.

With that logic, it doesn't take any focusing to increase the surface temperature of the source too (maybe only on a single atom scale)

cheers, Jamie

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Winfield Hill 9 years ago

Thanks, Rich, I've ordered a few to try out.

Thanks, - Win

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Martin Brown 9 years ago

But you still can't beat immersing the target object inside the source. That is the only time it sees 4pi steradians of blackbody radiation.

If what you think was possible then working perpetual motion machines would be on sale.

You really aren't listening are you?

Regards, Martin Brown

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tabbypurr 9 years ago

I've got it. File the top off a 5mm LED, and place right up against the target. Done :)

NT

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Alien8752 9 years ago

Correct, you can (theoretically) surround the Sun with fiberoptics and ai m them all on a point elsewhere, but the entendue never changes stays the s ame.

So? What happens next? You don't seem to have thought this through.

That only exposes the total surface area of the target to the local photo ns that would otherwise have entered the volume it occupies. The "idea" pro posed is to take the photons from a larger object's surface area and focus them on a smaller object, increasing the amount of energy it has to absorb in the same time, thus raising its temperature.

So, immerse the target in the source, but restrain the source from actual ly contacting the target while staying at the same temperature. Magnetic fi elds, whatever (a fusion reactor turned inside-out?). Imagine a 10 sq. mete r area sphere suspended in a 20 sq. meter surface area *bubble* immersed in the Sun. Keep increasing the bubble radius...

Use a large enough bubble and a small enough target in a Really Big Star and eventually you'd reach a target within which no matter could exist, not even quarks, effectively infinite temperature.

That appears to be the idea, anyway.

He's listening but he can't draw pictures in his head.

Mark L. Fergerson

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George Herold 9 years ago

aim them all on a point elsewhere, but the entendue never changes stays the same.

tons that would otherwise have entered the volume it occupies. The "idea" p roposed is to take the photons from a larger object's surface area and focu s them on a smaller object, increasing the amount of energy it has to absor b in the same time, thus raising its temperature.

ally contacting the target while staying at the same temperature. Magnetic fields, whatever (a fusion reactor turned inside-out?). Imagine a 10 sq. me ter area sphere suspended in a 20 sq. meter surface area *bubble* immersed in the Sun. Keep increasing the bubble radius...

r and eventually you'd reach a target within which no matter could exist, n ot even quarks, effectively infinite temperature.

There's an xkcd what if that covers this....

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(It comes with pictures...)

George H.

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Phil Hobbs 9 years ago

That sort of thing is what my thought experiment rules out, see above.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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