(3N+1)-frame phase retrieval for double-grating Ronchi lateral shearing interferometry

•A new shear-phase retrieval algorithm for double-grating Ronchi lateral shearing interferometry is proposed.•The algorithm allows for adjustable shear ratios without any hardware change.•The algorithm can reject unwanted diffraction orders, verified by the spectral analysis using its frequency transfer function.•Significant improvement on the accuracy of wavefront measurement is demonstrated in both experiment and numerical simulation.