A system and method for analyzing the characteristics of a thin film is provided whereby the in-plane birefringence of thin films is determined by measuring the interference fringes in the transmission or reflection spectra using unpolarized light and light linearly polarized along the MD and CD directions. The three spectra can be measured simultaneously or sequentially. The in-plane birefringence data can be used to characterize clear polymer films, which are principally made of biaxial oriented polymer, as the film is being continuously fabricated on a production line.