This paper presents an on-line optical device to monitor in-situ the polymer melting process in the extruder melt seal zone by diverting part of the melt flow out of the main stream to slit-dies. Measuring melt flow turbidity using unpolarised transmitted light intensity can quantitatively follow the polypropylene melting process in the melt pool zone. By using cross-polarized light the birefringence can be measured showing that 45° kneading elements cause chain orientation at extrusion temperature just above the PP melting point. The output flow rate of the diverted melt shows that the claimed forward pumping effect of 45° kneading elements is not present at the processing condition used here. Also the level of dispersion imposed by these elements can be estimated by measuring the area under the RTD curve obtained by dispersion polystyrene in the polypropylene melt flow. Optical properties has shown to be a valuable tool to characterize the level of fusion and dispersion of the second phase during extrusion.