ANYONE who has tried to look into a house or car on a sunny day can testify
that glass is not completely transparent. But new antireflective coatings
developed in Germany should make it possible to treat windows and solar
collectors so that they let in more light than even the finest of today鈥檚
optical instruments.
Ordinary glass reflects about 8 per cent of sunlight in the visible spectrum
because of the large difference in the refractive indices of air (1.00) and
glass (1.51). Optical glasses and camera lenses are coated with an alternating
series of films of high and low refractive index. These set up an interference
pattern that prevents incoming light from being reflected. 鈥淏ut they are quite
expensive, both literally and optically,鈥� says Dieter Sporn of the Fraunhofer
Institute for Silicate Research in W眉rzburg. 鈥淭hey are too costly for large
surfaces, and they prevent some of the light from getting through.鈥�
Instead, Sporn and his colleagues at the Fraunhofer Institute for Solar
Energy Systems in Freiburg have developed a coating made of silicon dioxide,
which creates a porous covering on the glass. The researchers dip the glass in a
coating solution of silicon dioxide and then bake it at 500 掳C. 鈥淭he coating
looks like a sponge consisting of a silicon dioxide framework riddled with pores
about 15 nanometres in diameter,鈥� says Sporn. Because these microstructures are
much smaller than the wavelength of visible light, the light enters the coating
as if it were simply air.
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鈥淭he silicon layer has an effective refractive index of 1.22,鈥� says Sporn.
鈥淭his means that for visible wavelengths, 99.5 per cent of light is transmitted,
while about 97 per cent of the solar spectrum passes through.鈥�
Sporn expects the first applications to be in solar energy. 鈥淪olar collectors
today use ordinary glass that reflects over 9 per cent of the incoming energy,鈥�
he says. 鈥淐utting this loss to about 3 per cent will mean a big gain in
efficiency at very low cost.鈥�
