The PM sensor uses a laser scattering principle, which works by using a laser to radiate suspending particles in the air. Then scattering light is collected in a certain degree, and finally the curve of scattering light change with time is obtained. In the end, equivalent particle diameter and the number of particles with different diameter per unit volume can be calculated by microprocessor based on MIE theory.
Functional diagram of the sensor’s operation (figure courtesy of ©Honeywell).