In order to reduce the difficulties in manufacture and assembly of the spatial filter with two square windows in image plane,we propose a grating lateral shearing interferometry based on a nine step phase shifting method,which is used to calibrate the zero-order phase error.The zero-order crosstalk could be eliminated only by a variable space filter.The parameters are designed and the principle verification system is completed for a projection lithography lens with numerical aperture of 0.125.Then the wavef...