A method to calculate the optical constants (refractive index n, film thickness d and extinction coefficient k) of
double-coated thin film by use of transmission spectrum is described in this paper. For double-coated glass films, the
relationship between T, λ, n, k and d is analyzed theoretically. Then the method to determine these optical parameters by
curve fitting is introduced. Several samples with different Sn concentrations have been prepared. According to the
method mentioned above, we calculate the coefficients of Sn-doped SiO2 films produced by Sol-gel method. Two kinds
of expressions for the refractive index n are introduced. One is Cauthy model. The other is defined as Polynomial model.
Also the expressions for k are given similarly. At last, the experimental curves are fitted according to the transmittance
formula. Results show that the refractive index becomes larger with the increase of Sn in the compound. Also, it turns out
that the refractive index decreases when the wavelength increases. This method combines the extreme point method with
the whole transmission spectra fitting method. This can improve the fitting accuracy.
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