[en] Water-droplet aerosols are of interest in many industrial processes, and their efficient removal from any gaseous effluent requires quantification of droplet concentrations and size distributions. Optical aerosol analysers such as the Polytech HC-15 can be used to monitor the number concentration and size distribution of such aerosols. However, the intensity of the scattered light detected is dependent on the refractive index of the aerosol droplets. A method has been developed to calibrate the Polytech HC-15 with water droplets that have been transported through a single-stage impactor whose characteristic 50% cut-off diameter (D₅₀) can be varied over the size range 3 to 18 μm aerodynamic diameter. The D₅₀ of a particular assembly can be related to the appropriate size channel of the Polytech by comparing the measured droplet-size distributions with and without the removable collection stage in position. (author)