In polymer processing or during the fabrication of polymer based products, the determination of quality measures is gaining more and more focus. In order to control the production process and to react with corrective measures when needed, it is required to determine those quality measures as early as possible in the process line. This can be seen as a generalization of the scope of the to be delevoped measurement system and this thesis. The developed, fluorescence based measurement system is intended to analyze the extrusion process. Especially the extrusion process of co-rotating twin-screw extruders is focused. The outcome of measurements are residence time density functions and their characteristics is explained and discussed. As the measurement systems is based on an optical principle, the metrological limits due to the absorption of light by the processed materials is a focused topic in this work. When the determination method and analyzis of residence time distribution functions is inspected more deeply, system theoretic models of the mixing behaviour of extruders are derived. Based on the system theoretic models, individual srew elements are characterized. Conversely, modelling of entire screw configurations by combining single srew elements is addressed. For investigating the characteristics of residence time density functions, mixtures being fed to the extruder are statistically modelled. This is to analyze the mixing of the output of an extruder, which is therefore modelled as a linear, time-invariant system. These steps are considered to be the core points of the thesis. As this work aims to be also a documentation and operation manual of the developed measurement system, performed measurements are presented and discussed.