The particle size distribution in single-particle counters is determined by comparing the detected pulse heights of the optical signals that correspond to the single particle flow through a small illuminated zone with a standard calibration curve, obtained from a set of uniform particles of known diameter.For determination of the concentration, the flow rate (the volume to be tested) is also measured simultaneously.The different light detection geometries applied in different particle counters yield a wide range of instrument designs and constructions.As the dependence of the scattered intensity on the refractive index of the particles is less pronounced around 90° of scattering angle, in most airborne particle counters perpendicular scattering geometry is implemented.A typical light scattering geometry is shown in Fig. 22.3. the aerosol to be tested passes through the illuminated zone and scatters the light of the laser beam in all directions.Part of this scattered light at a certain acceptance angle is collected by the optical system of the detector.The amplitude of the photoelectric signal generated on the detector by the particle crossing the illumination zone is compared with the standard calibration curve obtained from the set of known particles.