Fluorescence correlation spectroscopy examines the chemical and the photophysical dynamics of dilute molecular solutions by measurement of the dynamic optical fluctuations of the fluorescence of a few molecules, even averaging less than one molecule at a time, in open focal volumes that are usually less than a femtoliter (<10<sup>−18</sup> m<sup>3</sup>). It applies the same principles of statistical thermodynamics as does quasi-elastic light scattering. Molecular interactions, conformational changes, chemical reactions, and photophysical dynamics that are not ordinarily detectable by quasi-elastic light scattering can be analyzed by fluorescence correlation spectroscopy in cases in which molecular fluorescence changes in the dynamic range 10<sup>−7</sup>–10<sup>2</sup> s.
© 2001 Optical Society of America
(030.0030) Coherence and statistical optics : Coherence and statistical optics
(170.1580) Medical optics and biotechnology : Chemometrics
(180.0180) Microscopy : Microscopy
(270.2500) Quantum optics : Fluctuations, relaxations, and noise
(300.0300) Spectroscopy : Spectroscopy
Watt W. Webb, "Fluorescence Correlation Spectroscopy: Inception, Biophysical Experimentations, and Prospectus," Appl. Opt. 40, 3969-3983 (2001)