Intracellular accumulation and location of photosensitizers, e.g. 5-ALA induced protoporphyrin IX, are crucial parameters for the efficiency of photodynamic therapy (PDT). Fluorescence microscopy has proved to be a powerful technique to assess these parameters, however, even at low light doses around or below 1 J/cm² cells may be irreversibly damaged. Therefore, prior to microscopic experiments non-phototoxic light doses were determined, and experimental conditions of laser scanning (LSM) and wide field microscopy were adapted to these doses. Wide field images appeared more brilliant than LSM images, thus demonstrating some advantage of simultaneous over sequential detection. In addition, human glioblastoma cells appeared less sensitive towards illumination by an evanescent electromagnetic field than towards epi-illumination, since only their plasma membranes and adjacent parts were exposed to light.