Design of selective surfaces for solar energy collection

3umerical calculations have been done to determine α_THS, the total hemispherical solar absorptivity, and ε_TH, the total hemispherical emissivity in the infrared, of selective surfaces composed of a dielectric film on a metal substrate. The selectivity of the surface as a function of n ̂_L = n_L + ik_L, the refractive index of the film, is discussed, as well as the effect of grading the index of the film. It is concluded that the most effective film is one having n_L as close to unity as possible through its entire thickness while k_L increases from zero at the front surface to a larger value at the back surface.