Invented in the 1970s, laser granulometry is the technique used to measure particle or grain size between 0.05 and 900 µm (flour, powder, sand, etc).
Laser granulometry physics
The technique is based on laser light diffraction. A laser beam passes through powder transported by water or air. When a particle is illuminated by laser beam, one can observe diffraction fringes. The intensity of diffracted radiation, and the diffraction angle depend on particle size (the smaller the size, the larger the angle of diffraction). This is the way to obtain the "Airy disk" (a diffraction pattern, sort of a central disc and increasingly attenuated concentric circles). The width of the Airy disk will eventually help determine the grain size.
Measurements are very fast, a few seconds are required to read out 2000 to 4000 times the light scattered by thousands of particles contained simultaneously in the laser beam.
Laser granulometry
Courtesy of Jeremy and Noureddine - IUT Robert Schuman Illkirch
