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Hydrogen plasma reduction of fluorine doped tin oxide is a beneficial method to form tin nanodroplets on the sample surface directly in the plasma-enhanced chemical vapor deposition reactor. The formation of catalyst droplets is a crucial initial step for vapor-liquid-solid growth of silicon nanowires for radial junction solar cells and solar fuel cell technology. We present an original optical model which allows us to trace the formation process on fluorine doped tin oxide on soda-lime glass substrate from the in situ data and is in a good agreement with the spectroscopic ellipsometry data measured before and during the reduction process. The model reproduces well the phase shift introduced by a transition double layer in fluorine doped tin oxide which acts as a barrier against the sodium diffusion. Furthermore, we study the process of tin reduction from fluorine doped tin oxide in a real time and compare estimated amount of produced metallic tin with images from scanning electron microscopy.The proposed approach is very important for in situ real-time monitoring of the one-pump-down fabrication process used to grow nanowires and form radial junction devices.