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A bright idea for funneling sunlight.

A bright idea for funneling sunlight

In theory, an optical device should be able to concentrate sunlight to reach temperatures at the Earth's surface comparable to the sun's 5,800-kelvin surface Temperature. However, conventional lenses and mirrors, which are designed not only to concentrate light but also to form an image, fall far short of delivering the maximum possible concentration of sunlight. Now a team of University of Chicago researchers has invented an optical system that comes close to this thermodynamic limit. Their system concentrates sunlight at the Earth's surface by a factor of almost 60,000, offering the possibility of using solar energy to power lasers, destroy hazardous waste and process certain materials.

The trick is to gather light as efficiently as possible without trying to form an image. In such "nonimaging optics," it doesn't matter what paths light takes through the optical device so long as it all arrives at a single spot. "If you drop the imaging requirement and you simply collect light, you can reach the theoretical limit of concentration," says physicist Roland Winston, who led the research effort. Winston and his colleagues describe the design and operation of a concentrator based on this idea in the May 18 NATURE.

In principle, the entire solar concentrator could consist of a single, nonimaging optical element, but it would be too large and unwieldy for practical applications. In Winston's design, a parabolic mirror focuses light into a roughly cone-shaped "light funnel" that concentrates the light further (see diagram). The researchers make the funnel from a transparent material with a high index of refraction so that it both bends light and internally reflects any light that happens to hit its sides. Thus, practically all the light that enters the funnel exits through an aperture only 1 millimeter across.
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Publication:Science News
Date:Jun 3, 1989
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