Fig. 1. Diagram representing off-axis beams.

Fig. 2. Beam propagation in the (a) meridian plane and (b) the sagittal surface.

Fig. 3. The diffraction patterns obtained by the R–S (upper panel) and analytical (lower panel) formulas.

Fig. 4. (a) and (b) curves.

Fig. 5. Curve of .

Fig. 6. curves under different incident angles.

Fig. 7. Diagram of an obliquely incident spherical wave.