Fizeau interferometry for measuring refractive index and thickness of nearly transparent films

A simple method for measuring refractive index and thickness of nearly transparent films from Fizeau interferograms is described. The method requires no polarizing optics, variation of wavelength or angle of incidence, or precise light intensity comparisons. Commercially available Fizeau surface relief measuring instruments can be used without modification, the only necessary change being a partial, rather than complete, metallization of the sample surface. Analysis of the contributions of different sources of error to the over-all error in the refractive-index measurement indicates that the measurement is quite insensitive to weak absorption in the dielectric film and to finite absorption in the metal layer. For typical values of index and thickness, the thickness error is about 0.005 wavelengths, and the refractive-index error is of the order 0.01.