Particle counters are not counting particles
Light Scattering Airborne Particle Counters (LSAPC) are not counting every single particles physically but measures scattered lights from particle surface after passing by laser column inside sample path. The amount of light scattered by particle is related to the size of the particle. Equivalent diameter (D) can be calculated from the measured surface area (A) by D=2√(2A/4π) . Photo dedector collects this scattered light thanks to optical system design (concave mirror & lenses).