The determination of the complex refractive index (h) and the physical thickness (d) of very thin films (d congruent to lambda/50, lambda is the wavelength in VIS and NIR) is still a challenging task in the field of the inverse optical problems. The physical reality of these films makes difficult the application of methods, commonly used for the determination of (n) over tilde and d of thicker films. For several years we have been working on the development and the implementation of a method, designed especially for determination of h and d of very thin films. The nanothickness of the films allows us to develop in series of (n) over tilded/lambda the Abeles characteristic matrix elements. Thus, approximated expressions for the film transmittance (T-f), front side (R-f) and backside (R-f) reflectance are derived. For estimation of (n) over tilde and d we use the system (1+R-f)/T-f, (1-R-f)/T-f and (1-R'(f))/T-f. Here we apply an exact analytical approach to solve the system, obtained by development of the Abeles characteristic matrix elements to the 4-th power. For the first time, the set of equations for (1+R-f)/T-f, (1-R-f)/T-f and (1-R-f)/T-f is solved analytically. Besides the increase of the accuracy of the solutions, no initial information about the unknown parameters is needed.
