An aerosol generation system has a light source arrangement which provides signals at first and second wavelengths, and the detected light signals are recorded. The detected signals are processed to derive at least a measure of the aerosol particle size. This can be used in combination with the other parameters which are conventionally measured, namely the aerosol density and flow velocity. Thus, optical measurement (possibly in combination with an air flow measurement) can be used to estimate the aerosol output rate as well as the particle size.