Evaluation of the Refractive Index of Thin Transparent Polyethylene Terephthalatefilms

The properties of refractive index and thickness in thin transparent films were evaluated many years ago utilizing various optical techniques. Many of the more modern techniques involved the measurement of various film properties using interferometry. Until the early 1980s, interferometers were generally prohibited from routinely operating in the visible light region due to the requirements for exacting alignment of many of the optical components. Despite the wealth of literature, the alignment of films with the dielectric axes for measurements of the refractive index and also polarized vibrational analysis, have not been considered. Additionally, most of these publications assume either a film thickness or a refractive index referenced from others work. The work presented herein details several methods that can be used to determine both the film thickness and the refractive indices of polyethylene terephthalate films. Results indicate that the polarization axes only coincidentally lie along the draw axis; no film studied in this work had an axis along the draw axis. The refractive indices for the minor and major axes were found to be 1.61 and 1.63, respectively.