Calculated focusing efficiency (FE) of three different types of zone plate lenses. (a) The theoretical focusing efficiency of the TKZP as a function of the structural factor α, photon energy and lens thickness, based on the thin grating approximation. The map shares the same color bar with (c). (b) The focusing efficiencies of the FZP, KZP and TKZP (α = 0.2) calculated by the QDHT-BPM approach at 8 keV. (c) Focusing efficiency map in the plane of the structural factor α and the lens thickness t at 8 keV, obtained by QDHT-BPM.