Figure 1.

To achieve high diffraction efficiency across the entire pupil of a multilayer Laue lens, the layer periods d_n at heights h_n must follow the zone plate law such that reflected rays constructively interfere at the focus. The layers must be wedged so that Bragg’s law sin θ_n=λ/(2d_n) is satisfied locally at every bi-layer for a wavelength λ. For a lens of focal length f, this places the layers normal to a circle of radius 2f. The lens can be thought of as an off-axis portion of a larger parent lens. The numerical aperture is given by sin θ_NA, where θ_NA is half of the difference between the largest and smallest deflection angles.