We demonstrate that the thickness of a polymer film may be determined by a simple analysis of the interference fringes observed in a transparent region of its transmission spectrum. By measuring the transmission spectrum of the uncoated substrate in the same wavelength region, we can determine the dispersion of the refractive index for the substrate and film and the thickness of the film, without prior knowledge of the optical constants of any of the materials. The results of our approach agree with measurements obtained by a direct contact method. This analysis provides a fast, simple, and nondestructive method of thickness measurement using equipment available in many teaching laboratories. The method enables students to make routine thickness measurements while introducing them to refractive index dispersion, Fresnel coefficients, thin film interference, and regression analysis.
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