In this paper, we deal with an ellipsometric and infrared absorption spectroscopy (ES) studies of photoluminescent systems consisting in silicon nanograins grown within a silica matrix by thermal annealing after magnetron cosputtering of both silica and silicon chips. The modelling of the ES spectra used a novel approach based on the effective medium approximation, which allows us to estimate the values of excess silicon. These values were associated with the evolutions of both absorption coefficient and refractive index. The dielectric function of the inclusions was also determined and appeared to be dependent on their nanoscaled size. This aspect enabled us to closely correlate the deduced dielectric function to the visible photoluminescence (PL) of our Si/SiO2 systems, Both energy and efficiency of the PL were found to be closely governed by the rate of silicon incorporated within the silica matrix. Finally, the infrared spectroscopy investigations gave a clear account of the quality of the interface between the silicon nanoinclusions and the silica matrix, and hence its involvement in the PL features,