Optical properties of model 3D random composites

The measurements in the visible and near infrared of the optical constants of pellets of KBr filled with carbon black particles are reported. For very low carbon black volume concentrations, the extinction coefficient increases linearly with the concentration in agreement with the predictions of the effective medium theories. These theories fail to account for the non-monotonous concentration dependences of the real and the imaginary parts of the complex permittivities observed for every value of the wavelength. It is shown that the description of the optics of such random heterogeneous media by effective optical constants is valid as a first approximation.