The sample, dispersed in liquid or gaseous carrying media, passes through a monochromatic laser beam. The light is diffracted to different angles by the particles, and is measured by a multi-channel silicon detector. Using Fraunhofer's and Mie's work, CILAS has developed reliable, precise and fast computing algorithms. They enable to determine the particle size distribution from the diffusion pattern of the sample. The size distribution curve for the range 0.3 to 400 µm is available in one single measurement, without any mechanical adjustment or optical realignment.