FCS instrumentation for use in living cells. On the left hand, there is schematic FCS setup including laser excitation filters, emission filters, confocal pin hole and single photon detector (APD). To use this setup, the laser beam is positioned inside the cell (A). The exact position of the focus is established by performing a Z scan (B). The pin hole cuts out a defined focal element from the laser focus (C). The Fluorescence signals from fluorescenct entities moving through the focal element are recorded by the single photon detector, resulting in a Fluorescence trace (D). (Source: Shahzad A, Edetsberger M, Köhler G. Fluorescence Spectroscopy: An emerging excellent diagnostic tool in Medical Sciences. Applied Spectroscopy Reviews J (In press).