Abstract: A blade or thin beam of concentrated coherent light is formed in order to detect and calculate the number of particles present in a moving suspension that flows through a suspension conduit featuring a relatively small section and which is made of optically transparent material. This beam extends through the entire section at a reduced thickness and in the flow direction. A suitable detector detects and counts the particles that move across the blade, operated by the absorption of light that occurs when each individual particle passes through it.