Representation of the Ewald limiting sphere for monochromatic light of wave number k₀ in q – space. The q – values belonging to specular reflection (q_x = 0) and backscattering are marked. The most outlying edges at q_x = ±2k₀NA correspond to the highest resolvable spatial frequencies for the maximum incident angle θ_in,max. k_r is representing the reflected wave vector. As an example, the first diffraction orders for a periodic structure with a period length Λ were inserted.