Surfaces that selectively absorb solar radiation can be made by placing a solar absorbing assembly of layers that is transparent in the ir (X > 2 gim) on a substrate with a low ir emittance, such as a highly polished metal.Most of the selective surfaces currently used have been produced by adapting existing processes, such as the black chrome electroplating process,' or by refining older selective surfaces such as the chemical copper oxide on copper2' 3 or electrolytic black nickel.2 ' 4 These surfaces have been optimized experimentally. Certain features, such as surface roughness5 or grading the refractive index, are obviously beneficial to the operation of the surface, but little work has been done on the theoretical optimization of surfaces with respect to these and other parameters. For example, even for the simple case of a single uniform interference layer, the integrated solar absorptance has not been determined for optimum thickness as a function of n and k, the real and imaginary parts of the refractive index.