AbstractA new technique has been developed to measure the optical refractive indices in the parallel and perpendicular directions to the film surfaces and applied to measure the refractive indices of free‐standing polymer films. The refractive indices were obtained by measuring the reflectivity as a function of the incident angle. The angle dependence of the reflectivity which results from the interference of the light beam reflected from the front and rear interfaces of the film was analysed by means of the Fresnel equation. This technique can be used to measure the refractive indices in three major axes, i.e. one out‐of plane axis and two in‐plane axes of the sample. This technique gives values for the refractive indices and the thickness simultaneously with an accuracy comparable to that measured by optical wave‐guide technique. The validity of the technique has been tested with polymer films of known refractive indices such as poly(methyl methacrylate) and polyimide (PM
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