The dependence of the optical properties of p-doped porous silicon layers on the doping level of the substrate is studied systematically. Reflectivity measurements from the far infrared to the ultraviolet as well as Raman and photoluminescence experiments have been used as analytical tools. A clear correlation of the photoluminescence intensity and the strength of the Si-H stretching modes which monitors the surface-to-volume ratio in freshly prepared samples is shown.