Extinction coefficient of free-standing chitosan films determined from partially coherent transmittance spectra

Abstract The extinction coefficient (k) of free-standing chitosan-based films is determined in the spectral range 250–840 nm from normal incidence transmittance spectra showing partial coherence. Films of thicknesses (d) ranging between 3.7 and 20 μm casted from chitosan-acid acetic solutions are analyzed. Fast Fourier transform of oscillations in the transmittance spectra is used to determine film thicknesses by assuming normal dispersion in the refractive index (n) as reported in the literature. Transformation of partially coherent spectra to incoherent spectra was used to determine k which resulted of the order of 10−4. The strength of anomalous dispersion in n introduced by an absorption band in k centered at wavelength 293 nm is analyzed within the framework of the Lorentz harmonic oscillator model. The reliability of the assumed dispersion of n, the determined k spectra and d values is tested by analyzing the experimental transmittance spectra with a theoretical model accounting for partial coherence. An inverse relationship between the degree of coherence and film thickness is found.