Abstract: Spectrally selective absorption coatings for solar thermal energy conversion applications should exhibit a high absorptance in the 0.4 - 1 $mu@m wavelength range of the electro magnetic spectrum. Additionally, a low emittance in the thermal radiation range has to be attained. Niobium films displaying such optical properties were obtained by sputtering onto Si(100) and Mo substrates and suitable variation of the preparation parameters. The sputtered films were characterized by in situ photoelectron spectroscopy, and not in situ optical reflectometry and scanning electron microscopy (SEM). Emittance and absorptance were determined as function of temperature by the `dynamical vacuum technique.' It is found in particular that the as prepared films show a dendritic surface structure as revealed by SEM. In addition, the wavelength of transition from highly absorbing to reflecting character is shown to correlate with the inter-dendrite distance. !14
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