Diffusion is the final stage of diffraction by objects smaller than the wavelength and arranged randomly enough that there is no, or virtually no, phase relationship between the waves diffracted by each of them. The directing parameter of scattering phenomena is the ratio of the wavelength to the size of the diffracting object, or size parameter. Several theories are attached to the phenomenon, depending on the value of this parameter and the concentration of diffusers. Although surface, and therefore, two-dimensional scattering can occur, it is essentially a three-dimensional phenomenon. Historically, processing has taken place within the wave framework of electromagnetism (Fig. 12.1).