“…A number of extremely sensitive fluorescence detection techniques are available based on registering single photons (Papageorgas et al, 1999;Tiberg and Paulauskas, 1981;Evtyushenkov et al, 1982;Shelevoi, 1985) commonly referred to as the single photon detection (SPD) techniques. Because of their complexity and cost, in biomedical applications single photon detection techniques are mostly used for time resolved fluorescence spectroscopy or detection of single fluorescent molecules (Dovichi et al, 1984;Chen and Dovichi, 1996;Soper et al, 1992Soper et al, , 1993.…”