A numerical model was developed which enables the calculation of the optical constants (refractive index, n and extinction coefficient, k) of thermochromic coatings based in undoped and doped vanadium dioxide thin coatings deposited on glass for use as an intelligent window - a window that can change the optical properties in response to the temperature. From experimental results it can be seen that the vanadium dioxide coating prepared by Atmospheric Chemical Vapour Deposition shows a switching efficiency of about 30% at 2500 nm. In the visible range the transmittance and the reflectance does not change with the temperature both for the undoped and Nb doped VO2. For the Nb doped vanadium dioxide coating the switching efficiency is about 20% at 2500 nm. From the numerical simulations a n=2.89 and k=1.33 above Tc and n=2.39 and k=0.52 below Tc (at wavelength of 2500 nm) were determined for the undoped vanadium dioxide coating. The Nb doped vanadium dioxide coating calculations results on n= 2.45 and k=1.56 above Tc and n=1.92 and k=0.88 below Tc.