An analysis of interference fringes (channel spectra) in transmission
or reflection spectroscopy of optical films can provide an accurate me
thod for determining the refractive index or thickness of the film. If
the index is changing and the common equation n = 1/(2tDELTAnu) deter
mined from the positions of two adjacent maxima in the interference pa
ttern is employed, the resulting refractive index does not lie between
the correct values at the frequencies of the two maxima as intuitivel
y 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 li
near dependence on wavelength.