The determination of the values of the optical constants (n, refractive index and k, extinction coefficient) in the visible range (400-700 nm approximately) of both silver (Ag) and tin oxide (SnO2) films is a necessary condition to predict the behavior of a SnO2/Ag/SnO2 multilayer low emissivity coating in relation to visual comfort. For this purpose, thin films of Ag and SnO2 with a thickness similar to that in multilayer coatings (90 and 380 Angstrom, respectively) were used in order to avoid the thickness dependence of the optical constants. A simulation computational program based on the matrix method was employed to determine the values of the optical constants of these films in the visible range. It has been demonstrated that for the design of optical coatings with optimal properties it is essential a great accuracy in the determination of the optical constants, since a minimum variation of these values leads to a deterioration of the behavior of the whole coating.