light and other photons is most commonly studied via correlation methods, in which the quantity measured directly is the intensity-intensity time correlation function $$S(q,t) = \langle I(q,\tau) I(q, \tau+t) \rangle,
     \label{eq:Sqtdef}$$ Here $q$ is the magnitude of the scattering vector, $I(q,\tau)$ and $I(q, \tau+t)$ are the scattering intensities over short time intervals near $\tau$ and $\tau+t$, and the brackets $\langle \cdots \rangle$ represent an average. Scattering is said to be due to scatterers within the medium, scatterers generally being represented mathematically by points whose locations are known. Scattering from extended bodies, such as high-molecular-weight polymer chains, is often treated by representing the scattering body as a series of scattering points whose relative positions are partly fixed. If the volume being observed is much larger than the volumes over which particle positions and displacements are correlated, quasielastic scattering corresponds to the intermediate structure factor (or field correlation function) $g^{(1)}(q,t)$ via[@crosignani1975a] $$S(q,t) = A |g^{(1)}(q,t)|^{2} + B.
     \label{eq:Sqg1def}$$ In this equation $A$ and $B$ are constants determined by details of the experimental apparatus; these constants have no effect on the time dependence. Homodyne rather than heterodyne detection of the scattered light is assumed. The factorization of $S(q,t)$ into $g^{(1)}(q,t)$ is sometimes termed 