Theoretical determination of the optical constants of weakly absorbing thin films

Methods for determining the optical constants of thin films from photometric measurements of reflectance and transmittance have been known to fail when applied to weakly absorbing films, because of the ill-conditioning of the equations used in the calculation. A method is presented which overcomes this problem by assuming that the refractive index obeys a dispersion formula of the Sellmeier type and then solving the equations simultaneously over a range of wavelengths. This method has been used to determine the optical constants of various materials in the near-ultraviolet and visible regions of the spectrum; results are given for the oxides of thorium, aluminium, zirconium and neodymium, together with neodymium fluoride.