Measurement of spectral refractive indices and thickness for biaxial weakly absorbing films. Application to stretched poly(ethylene terephthalate) (PET) sheets in the IR

Abstract Anisotropic refractive indices of many plastics with potential optical applications are difficult to find in spectral format. We propose a method to obtain dispersive biaxial refractive indices of films which is useful when absorption bands are present. Only spectral scans at different angles of incidence are used. Characterised for its robustness and reliability the method is suitable for the whole UV–IR range. It can be applied for any interferential thickness d provided we measure spectral intervals greater than approximately ∼ 0.2 μm . Absorption bands, typical of the IR for organic compounds, are subtracted from the different scans by measuring the transmission near the Brewster angle where no interference appears. We obtain the thickness and the principal refractive indices in the 1.3–5.7 μm spectral range of a poly(ethylene terephthalate) (PET) fully biaxially stretched film. PET have a great potential in high performance polarisation optical applications due to its high anisotropy (similar to calcite), a wide transparency range ( λ∼0.32–5.70 μm ) and its flexibility in optical constant tailoring depending on stretching ratios.