The focus of this manuscript is the investigation and visualization of cell damage and death caused by PDT on a subcellular level. For this purpose, three different dyes, the chemical structures of which are shown in Fig. , were employed. The first fluorescent probe is hypericin, which acts as photosensitizer for PDT. The second dye, resorufin, is mainly released into the surrounding medium and only a small amount remains in the cell's cytoplasm. Thus, it enables to draw conclusions about the cell membrane integrity and indicates if the surrounding medium is entering the cell. The third fluorescent probe is propidium iodide (PI), which is only fluorescent when intercalated into DNA/RNA. Furthermore, it is unable to diffuse through intact cell membranes and can only enter the cell/nucleus in case of decreasing membrane integrity or even rupture. Therefore, PI mainly functions as an indicator for cellular damage and death. These three dyes have very distinct optical properties. The respective fluorescence spectra and lifetime histograms are presented in Fig. . All three dyes have different fluorescence emission maxima (584 nm resorufin, 600 nm hypericin and 622 nm PI), however, there is a significant overlap between the individual spectra making it difficult to distinguish the dyes only based on the fluorescence intensity. In contrast, the fluorescence lifetime strongly differs for these dyes, where resorufin has the shortest lifetime of 2.8 ns, followed by hypericin with 6.8 ns and PI with a long lifetime of 12 ns in complete cell culture medium.