This part of ISO 21501 describes a calibration and verification method for a light scattering airborne particle counter (LSAPC), which is used to measure the size and particle number concentration of particles suspended in air. The light scattering method described in this part of ISO 21501 is based on single particle measurements. The typical size range of particles measured by this method is between 0,1 µm and 10 µm in particle size. Instruments that conform to this part of ISO 21501 are used for the classification of air cleanliness in cleanrooms and associated controlled environments in accordance with ISO 14644-1, as well as the measurement of number and size distribution of particles in various environments. The following are within the scope of this part of ISO 21501: — size calibration; — verification of size setting; — counting efficiency; — size resolution; — false count rate; — maximum particle number concentration; — sampling flow rate; — sampling time; — sampling volume; — calibration interval; — test report.