**Normal Diffusion**

The fluorescent particles used in FCS are small and thus experience thermal motions in solution. The simplest FCS experiment is thus normal 3D diffusion, for which the autocorrelation is:

where is the ratio of axial to radial radii of the measurement volume, and is the characteristic residence time. This form was derived assuming a Gaussian measurement volume. Typically, the fit would have three free parameters—G(0), and --from which the diffusion coefficient and fluorophore concentration can be obtained.

With the normalization used in the previous section, *G*(0) gives the mean number of diffusers in the volume

where the effective volume is found from integrating the Gaussian form of the measurement volume and is given by:

- gives the diffusion coefficient:

Read more about this topic: Fluorescence Correlation Spectroscopy, Interpreting The Autocorrelation Function

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