A laser beam emitted by the laser becomes a parallel beam after filtering, expanding, and quasi value. When no particles are illuminated, the parallel beam passes through the Fourier lens and
converges on the focal plane to form a small, bright spot - focal point, as shown below:
When the parallel laser beam meets particles, the particles will cause the laser to scatter and some of the light will scatter outwards at an Angle with the optical axis, as shown in the figure above.
It has been demonstrated by both theory and experiments that the scattering angle is small for large particle and the scattering angle is large for small particle. The scattered light from different angles will form a series of rings on the focal plane after passing through the Fourier lens. The bright and dark alternate light spots made up of these rings are called Airy plaques. Airy plaques contain rich particle size information. Simply speaking, the light circles with larger radius correspond to smaller particles, and the light circles with smaller radius correspond to larger particles; the light intensity at Airy plaques indicates the number or content of the particles.
Thus, a serial of photoelectrical detectors can be installed at the focal plane, which translates light signals to electric signals and input into a computer. Particle size distribution can then be obtained based on Mie scattering theory and inverse algorithm.