Abstract : A laboratory polar nephelometer was developed in order to measure the light scattered by particles along two directions of polarization. The minimal concentrations of detection and the uncertainty of measurement were estimated. A lookup table method was developed in order to retrieve the complex index of refraction and the size distribution of spherical particles from nephelometer data. It is shown that only the real part can be quantified for low absorbing particles. If the particles are strongly absorbing, only the imaginary part can be found. Otherwise, the method retrieves the complex index of refraction and the size distribution. The method was applied to real laboratory measurements and to data collected in cloud (2 clouds, 1 contrail) with an airborne nephelometer. The method provides good performance. It was shown that the particles of the contrails were not spherical water droplets.