A quantitative relationship between polarization properties of fluorescence light and molecular rotational diffusion was first derived by Perrin (1926). His results, which concerned spherical particles, have later been refined to the more complex rotational motion of asymmetric bodies (Memming, 1961; Chuang & Eisenthal, 1972; Ehrenberg & Rigler, 1972; Belford, Belford & Weber, 1972).

In the initial period of application of photon correlation spectroscopy to molecular phenomena, attention has mainly been focused on the improvement of experimental approaches and instrumental techniques, on feasibility studies of certain cases where theoretical considerations seemed to indicate the prospective usefulness and on various applications that are relatively straightforward, such as the measurement of translational or rotational diffusion coefficients, or the determination of flow velocity by Doppler anemometry.

About 11 years ago Ford & Benedek (1964) and Cummins, Knable & Yeh (1964) developed some fascinating new light-scattering techniques for the study of macromolecular motions in solution. Enthusiastic researchers have over the years endowed these new methods with a variety of colourful names, including optical mixing or light-beating spectroscopy, heterodyne or homodyne beating, intensity fluctuation spectroscopy, Rayleigh spectroscopy, and quasielastic or dynamic light scattering.