This paper presents a laser light scattering apparatus which measures the physical characteristics of a filter testing aerosol in its actual flow by analyzing simultaneously the intensity of the light scattered at different angles by each particle passing through an optically defined scattering volume. Both size and refractive index may be obtained provided that the incoming light flux is kept constant, within 15%, inside the scattering volume. The complete simulation of the optical apparatus was performed by supposing that only one particle at a time is analyzed and that the particles are spherical, thus allowing the Mie theory to model their scattering behaviour. Particle size, complex refractive index (real and imaginary part), and light flux incident onto the particle were considered as the four variables. Preliminary trial runs, performed with the aim of determining the minimum number of collecting angles necessary to obtain the convergence of fitting procedure to the expected values of the unknown parameters, showed that this convergence could be only occasionally obtained even if up to 10 collecting angles had been adopted. This occurrence demonstrated the existence of a strong correlation between the four parameters. Subsequent runs performed for non-conductive materials gave, on the contrary, better results.