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Absorption probabilities of evanescent waves in thin samples are calculated. The influence on absorption probabilities of the principal parameters characterizing the media and the radiation are studied by both theoretical and experimental means.

We examine the problem which occurs when distinct absorption bands are compared. This problem is linked to the variation of the analyzed thickness of the sample according to the wavelength of the incident beam. Theoretical and experimental corrections are proposed to solve this problem. The theoretical correction is next used to calculate the degree of crystallinity of three polyethylene terephtalate films which cannot be determined by the usual method of transmission spectroscopy.