ERRORS IN FILM REFRACTIVE-INDEX DETERMINATIONS FROM INTERFERENCE-FRINGES

An analysis of interference fringes (channel spectra) in transmission or reflection spectroscopy of optical films can provide an accurate method for determining the refractive index or thickness of the film. If the index is changing and the common equation n = 1/(2tDELTAnu) determined from the positions of two adjacent maxima in the interference pattern is employed, the resulting refractive index does not lie between the correct values at the frequencies of the two maxima as intuitively expected but well outside this range. This error can be large and is not restricted to the previously discussed case of an index with a linear dependence on wavelength.