The refractive indices and the extinction coefficients of Al doped a-Si:H films are determined from their transmittance spectrograms over a wide energy range, 0.45-2.5 eV. Analysis of the refractive index data yields the values of the long wavelength dielectric constant, the average oscillator wavelength, average oscillator strength, average oscillator energy, dispersion energy, and lattice energy. The change of the absorption edge versus photon energy shows the transition to be of the indirect type and gives as well the Urbach energy. The real and imaginary parts of dielectric constant are used to calculate the free carrier plasma resonance frequency, optical relaxation time and the ratio of free carrier concentration to the free carrier effective mass.