In this work a method for the determination of the thickness and optical properties of low optical thickness films (D < 600 nm), from the experimental transmittance (T) and back reflectance (R') measurements, is proposed. An original analytical formulation without any kind of approximation is employed for the computations. The computing algorithm is presented in detail and an error study is given. The method is applied for the characterization of sputtered CdTe and a-Si thin films of optical thickness in the range of 250 nm.