This paper reported a method to measure moisture swelling of amorphous PET films using Rayleigh interferometer. Plots of the optical path of light transmission in the polymer vs soaking time were obtained

and from the same curves

time dependence of the moisture swelling

saturated (equiliorium) water apsorption and diffusion coefficient of PET films were estimated. These results are in agreement with those obtained by the weighting method. It is shown that optical interference method is a simple but sensitive method for the study of warer absorption and swelling in transparant polymer films.