We report on the inline measurement of the residence time and residence time distribution (RTD) using fluorescence spectroscopy [1] versus color measurement. The measurements were conducted on different single and twin screw extruders, using thermoplastics and thermosets such as polyolefines, EVOH and polyester resins. To assess the influence of different screw configurations, screw speed, mass throughput and the degree of filling, on the RTD, tracer particles were inserted in the feeding section and the RTD was measured inline in the plasticization unit. Using a special measurement probe, which can be inserted in the 1/2’’ UNF bore holes, the mixing ability of either the whole screw, or selected screw regions, e.g. mixing parts, can be validated. The most common method for the determination of the residence time is to insert colored master batch. The sensibility of the used color sensor is exactly adapted to the perceptivity of the human eye and gives the color intensity of the polymer melt. In comparison, the fluorescence spectroscopy shows the real amount of particles and therefore the concentration of the used tracer particles. This makes it possible to detect the amount of particles as a function of time and the residence time distribution including the minimal, average and maximal residence time. Keywords: RTD, residence time distribution, extrusion, single screw, twin screw, fluorescence spectroscopy, color 1. G. Gerstorfer, A. Lepschi, J. Miethlinger, B. G. Zagar, An optical system for measuring the residence time distribution in co-rotating twin-screw extruders, J. Polym. Eng. 33 , 683-690 (2013)