More formulae relating to optical reflection and transmission by thin films

In order to derive refractive indices and coefficients of absorption from reflectance and transmittance measurements it may be necessary to take into account the effects of surface layers. This requires the use of formulae for a double film supported on a substrate. It is shown that the very complicated formulae for reflectance R 2 and transmittance T 2 for normal incidence on such a double film may be replaced by expressions for (1 ± R 2 )/ T 2 which can be expressed explicitly in terms of the n and k values for each layer. These formulae are relatively simple and very convenient for computing purposes. It is also shown that under certain circumstances a double layer may be replaced by an equivalent single layer.