Scattering vector ($\mathbf{q}={\mathbf{k}}_s-{\mathbf{k}}_i$)

The scattered minus incident wavevector. In elastic optical scattering, $q=(4\pi n/{\lambda }_0)\sin (\theta /2)$ for scattering angle $\theta$, refractive index $n$ and vacuum wavelength ${\lambda }_0$. A dilute diffusing-particle field correlation decays at rate $D{q}^2$ in the single-scattering limit.