Optical properties of smokes have to be known because of their influence on visibility which can be reduced during evacuation of people and firemen intervention. Measurements have been carried out in non-flaming condition on smokes produced by solid materials exposed to an intensive radiation flux of 25 kW·m-2. Two materials have been studied: wood and PMMA. A smoke chamber has been used to perform measurements of transmittance and optical density of smokes on the visible wavelength spectrum as recommended by standards. To go further, spectral measurements and analysis of the smoke optical properties for wavelengths between 350 and 1125 nm have been carried out. In parallel, the mass concentration of the particles has been measured and it allows to determine the specific extinction coefficient. The spectral measurements have highlighted the dependence of the transmittance, the extinction coefficient and the specific extinction coefficient on the wavelength. The main features of the spectral results are discussed and compared to other studies. A mean value of the specific extinction coefficient on the visible spectrum is proposed and compared to other data.