The growing demand for active safety systems implies also the requirement for new and suitable evaluation and test processes for such systems. However, the evaluation of these systems is in most cases a non trivial, time consuming and expensive task. Especially, for active pedestrian protection systems, which are based on infrared detection of humans. In this paper we present a simulation of Thermopile sensors to evaluate the performance of an infrared based pedestrian detection system. By means of the simulation tool, any test scenario can be generated and the simulation provides the sensor data to the generated scenario. The simulated sensor output depends on the objects placed in the virtual environment and the background radiation. Considering the physical sensor dimensions and Planck's radiation law, an approximate image of the reality can be achieved. Thereby, it is possible to simulate pedestrian crash tests, which saves extensive time of testing and enables a fast optimization and adaption of the system.