Combining Differential Images by Inverse Riesz Transformation

A method forms a representative image from a crossed grating fringe pattern interferogram of an object from an interferometer. The method determines a plurality of spectral lobes from the interferogram and selects from the plurality of determined lobes, two substantially orthogonal sidelobes. The selected sidelobes represent spatial differential phase information of the interferogram. The method applies an inverse Riesz transform to the spatial differential phase information of the selected sidelobes to form a transformed differential phase image, and forms from the transformed differential phase image, a representative image emphasising high frequency detail information of the object.