Fluorescence Photon Correlation

Abstract

In 1904 Smoluchowski derived¹ that the number N of particles in an open volume — i.e. one defined by optical beam geometry, not by mechanical walls — shows fluctuations δN the relative deviation of which is σ_N/N_o = N_o ^−1/2. As a consequence the fluctuations are discernible only if N_o is small. Verification of this by meticulous observations under a microscope is due to Svedberg who also coined the term paucimolecular phenomena². The phenomenon has been rediscovered in dynamic light scattering where it was referred to as occupation number fluctuations³. Recently, Magde, Elson, and Webb^7,8 combined the number fluctuation concept with analog fluorescence autocorrelation^4,5 into what they called concentration correlation spectroscopy.