Transmission of porous silicon (PS) has been measured using a new and direct method based on photo-induced current modulation in the silicon substrate. A current is induced in bulk silicon by electrical polarization on the back of the substrate. When the PS layer is illuminated, the light not absorbed in the layer and therefore transmitted to the silicon substrate modifies this current. In this configuration, the porous silicon layer is electrically passive. It is shown that the photo-induced current modulation strongly depends on the Al/Si contact used to polarize the silicon substrate. Al/Si contacts are responsible for the non-linear response of the system, which was overcome by working at low temperature and by using CW illumination on the back of the sample. important advantages of this technique lie in the fact that removal of the PS layer is not necessary and that scattering of light at the interfaces does not affect the measurement. Using this method, absorption coefficients of low- and high-porosity PS layers have been measured in an energy range starting from the near IR to the UV. The absorption spectra show an exponential behavior at low energy, characteristic of amorphous material, but at higher energy a saturation is observed as in the case of bulk silicon.