This document specifies characteristics of a light scattering aerosol spectrometer (LSAS) which is used for measuring the size, number concentration and number-based size distribution of particles suspended in a gas. This document provide the calibration procedure and the validation method for aerosol spectrometers. This document applies to: —    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 or composting facilities, nebulized droplets, measurements in street tunnels; —    fractional separation efficiency determination of filters.