Selective surface

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In solar thermal collectors, a selective surface is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio of radiation-absorption (alpha) to radiation-emission (epsilon),. Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface.

Normally, a combination of materials is used. One of the first selective surfaces investigated was simply copper with a layer of black cupric oxide. Black chromium ("black chrome") nickel-plated copper is another selective surface that is very durable, highly resistant to humidity or oxidizing atmospheres and extreme temperatures while being able to retain its selective properties - but expensive. Another combination consists of steel plated with gold, silicon, and silicon dioxide.

Although ordinary black paint has high absorption, it also has high emissivity, and thus its selectivity is low. So it is not considered to be a true "selective surface".

Typical values for a selective surface might be 0.90 absorption and 0.10 emissivity. but can range from 0.8/0.3 for paints on metal to 0.96/0.05 for commercial surfaces and emissivities as low as 0.02 have been obtained in laboratories.