Heat transfer occurs via radiation, conduction, and convection. When the radiative exchange is significant - which holds true for many important applications - one can achieve energy efficiency by exploiting surfaces with spectral selectivity. This term is taken to imply that the radiative properties (i.e., the absorptance A, emittance E, reflectance R, and transmittance T) are qualitatively different within different parts of the electromagnetic spectrum. Spectral selectivity can be obtained by use of coatings onto substrates that are either transparent or opaque metallic, or by treating the surfaces of the substrates.
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