Simple and accurate measurement of the absorption coefficient of an absorbing plate by use of the Brewster angle

We propose a simple and accurate method for measuring the refractive index and the absorption coefficient of an absorbing parallel plate. The method is essentially based on transmittance measurement with minimum reflectance at the Brewster angle for p-polarized light, which enables us to measure a low absorption loss of the order of 0.1 dB/cm for a typical plate thickness of 1 mm. The error that is due to the approximation used is practically negligible, and the accuracy of measured results depends mainly on the variations in the flatness and parallelism of the two surfaces of the sample and on instrumental errors.