20% of the PL intensity of the spots measured in the ER (see inset in the top right plot in Fig. ) and only small amounts of hypericin, resorufin and PI can be found. The spectral composition suggests that the main fluorescence component is hypericin, followed by resorufin. After M5, the PI fluorescence intensity strongly increases, until hardly any signal of hypericin and resorufin are visible, suggesting strong damage to the cell and ultimately cell death. Fluorescence spectra acquired in the large protuberance after M4 (Fig. ) are shown in Fig. . Immediately after bleb formation in M4, the spectra closely resemble the spectrum of a combination of resorufin and hypericin, which is validated by the spectral composition plot. This implies that the initial protuberance is filled with the cell's cytoplasm, within which both dyes are found to contribute approximately equally to the overall PL decay. In the next measurement (M5), however, the membrane stability appears to be compromised as the composition changes towards higher contributions of resorufin and PI, which are entering the damaged cell. Over the course of PDT, the hypericin fluorescence intensity decreases, whereas resorufin only shows small changes. The main indicator of the overall cell damage is again PI, which shows a strong increase of its fluorescence intensity at the later PDT stages. Finally, the fluorescence spectra acquired from the medium outside the cell are shown in Fig. and are mainly dominated by resorufin. Only at the last PDT stages, a small amount of hypericin and PI are observed here, since they are released into the medium due to the damaged cell membrane.