Minimal-Invasive Fluorescence Microscopy
Ultra-low excitation light (nW) and ultra-low label concentration (1ethidium bromide in 10.000 DNA base-pairs)  is required to preserve the living state in fluorescence microscopic cell studies. Photodynamic reactions can occur even at very low excitation levels . DASPMI-labelled mitochondria may serve as an example. Ultra-sensitive imaging detectors (QA-MCP-PMT) are required that have time-resolution < 10 ps at a dynamic range of > 106, or even > 107 with cooled photocathode and hot MCP-technology.
 M. Tramier, K. Kemnitz, C. Durieux, J. Coppey, P. Denjean, R.B. Pansu, and M. Coppey-Moisan, Restrained Torsional Dynamics of Nuclear DNA in Living Proliferative Mammalian Cells, submitted 1999.  Maite Coppey-Moisan et al.
-Nature is Time-Correlated-
Static intensity technology (CCD) is blind towards the huge pool of information, residing in the environment-sensitive fluorescence lifetime of a molecular probe. In addition, time-correlation of the emitted fluorescence leads to dramatic reduction of background, which is non-correlated in respect to the pulsed laser excitation. Application of the unique time-correlated single photon counting (TCSPC) technique , and its recent variant of time-and space-correlated single photon counting (TSCSPC)  in cell biology fluorescence studies, increases sensitivity by 10-100 times compared to steady-state CCD technology.
 D.V. OConnor and D. Phillips, (1984) Time-Correlated Single Photon Counting, Academic Press, New York.  K. Kemnitz et al.