For material research and device design of amorphous silicon solar cells, it is required to determine the refractive indices, thickness and absorption coefficients of a-Si films. Taking interferences and substracte effects into account, we propose a simple analytical method that allows us to determine the thickness, refractive index and absorption coefficient of a film from the usual transmittance spectrum at normal incidence. As an example, we calculated the optical gap of a-Si:H and a-SiC:H films.