Figure 1.

FCS. (a) Fluorescence is collected from the molecules (circles) in a small well defined volume (red) near the focus of a laser beam (pink) within an equilibrated solution. The number of fluorescent molecules (yellow) in this volume fluctuates due to diffusion/flow in and out of this volume and due to chemical transformation to and from a nonfluorescent (black) species. The fluorescence fluctuates (b Upper) in proportion to the number of fluorescent molecules in the volume, and its autocorrelation (b Lower) yields the dynamic parameters.