Correct interpretation of spectral interference measurements of weakly absorbing films of micrometer thickness

We present analytical equations based on thin-film optics with which to extract the thickness of weakly absorbing films from spectral interference measurements, using only the observed constant interference frequency. It is shown that the assumption of a constant index of refraction n for the analysis of interference frequency omega 0 introduces significant errors into the thickness calculation. Instead, the first derivative of n with respect to the energy, n', has to be included as well to yield the correct film thickness, even for small values of n'. The equations presented can be used as constraints in appropriate numerical methods to improve the film thickness iteratively.