Abstract

The error in the calculated refractive index of an absorbing dielectric film, which is determined by approximate equations that neglect absorption, is influenced by the true refractive index of the film and the controllable parameters—substrate and measuring wavelength. For a given absorption coefficient (k) the error is greatest for low refractive index. In tests over the visible spectrum and an assumed n = 1.4 and k = 0.01, Al and Ag give an error >100Å. The error using Cu and Au is small below a transition point near 5000 Å (about 10Å) and increases sharply thereafter, so that an absorption coefficient of 0.001 can give appreciable error above the transition point. Cr and steel are relatively independent of wavelength although they give an error somewhat greater than Au or Cu below the transition wavelengths of the latter. The approximate equations which neglect absorption give two relations for calculation of thickness. It is shown why one of these equations gives an insignificant error when small absorption is present and an independently determined refractive index is used in the thickness calculation.

© 1965 Optical Society of America

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