This article presents melt temperature measurements with an infrared thermometer during extrusion and investigates the influence by the surface emissivity and the rotating screw. This influence can be used for on line residence time measurements in the extruder line, to measure the number of revolutions of the screw, and to measure both the bulk＇s temperature. The result shows that colouring the PE with a high emisffisivity tracer allows us to measure the melt＇s surface temperature and minimise the influence of the rotating screw. Its influence is weaker in complete filled section as in partial filled sections of the extruder and the mean bulk＇s temperature is measured when the emissivity is lower than one. Furthermore it is shown that it is possible to measure the residence time distribution on line with an infrared thermometer, by taking advantage of its sensibility on a changing surface emissivity. Measurements with classical methods shows the same residence time distribution as obtained with the infrared spectroscopy. The advantage of this method is its wide range of applications, such like as the use in pressure zones, for a wide range of temperatures and to measure on line the local residence time distribution all along the screw.