This part of ISO 21501 specifies characteristics of a light scattering aerosol spectrometer (LSAS) which is used for measuring the size, number concentration and number/size distribution of particles suspended in a gas. The light scattering technique described in this part of ISO 21501 is based upon single particle measurements. The size range of particles measured by this method is between approximately 0,06 µm to 45 µm in diameter. Instruments that conform to this part pf ISO 21501 are used for the determination of the particle size distribution and particle number concentration at relatively high concentrations of up to 1011 particles/m3. Application fields include: — characterization of metered dose inhalers (MDI), dry powder inhalers (DPI) and nebulizers in pharmacy; — production control of active agents; — cut-off determination: impactors, cyclones and impingers; — atmospheric aerosols: bio-aerosols, stables/composting facilities, nebulized droplets, measurements in street tunnels; — fractional separation efficiency determination of filters. For the above-mentioned applications, aerosol spectrometers should determine the particle size distribution, particle number concentration, size resolution and sizing accuracy as accurately as possible. These aerosol spectrometers are not suitable for the classification of clean rooms.