Optical constants and spectral selectivity of stainless steel and its oxides

Reflectance measurements on eleven grades of stainless steel have been performed. It is found that the metallurgical phase is a more important parameter for the reflectance behavior than the detailed content of the alloying elements. The optical constants for one austenitic and one ferritic grade have been calculated by the Kramers-Krönig analysis. For the extrapolation, a fitting procedure to the results from measurements with polarized light at 10° and 60° incidence was used. The austenitic steels have a higher reflectance than the ferritic and martensitic, but their integrated solar reflectance of 68% is considered too low for solar reflectors. Two of the natural oxides on stainless steel, Fe2O3 and Cr2O3, have been studied as separate films, and their optical constants determined by combined transmission and reflectance measurements. The metal and oxide results are combined in an evaluation of the optical selectivity that can be obtained with oxidized stainless steel surfaces. It is concluded that a ferritic steel base is to be preferred because of its lower reflectance. Assuming perfect boundary layers and using conventional interference calculations it is found that Fe2O3 gives better selectivity than Cr2O3 since both the first interference minimum and maximum will be lower in that case.