 Coherent continuum phase matching

In a homogeneous medium driven by a plane wave, coherent scattering amplitudes carry phases $e^{i({\mathbf{k}}_i-{\mathbf{k}}_s)\cdot \mathbf{r}}$. An infinite uniform volume cancels arbitrary nonforward components. Finite shapes, boundaries and fluctuations determine any side signal; summing independent-particle intensities is not the continuous-medium limit.