The measurement of transmission is an often used measuring principle in technology. The evaluation algorithms of fluctuating transmission signals are well known in the case of single particle measurement. By evaluating fluctuating transmission signals the particle characterization is also possible at high particle concentrations (0.01 to 30 vol.-%). In comparison with conventional photometers a significant gain of information can be received. Fluctuating signals are caused by the statistical probability of a limited number of particles being present in a defined measuring volume. The evaluation of such signals allows the determination of particle concentration, particle size of monodisperse particles (extinction diameter) and information about structure of agglomerates independent from each other. Mathematical fundamentals and practical ways to measure these parameters are shown. Experimental results are examplarily presented for monodisperse, polydisperse and agglomerated suspensions.