A differential diffractive interferometric technique allows the high resolution retrieval of the spatial frequency profile along a long grating of supposedly constant period from the measurement of the phase of the two interference products detected by a double read head sensor upon its fly over the grating under test. The application of the technique to a 100 mm long grating written by a fifth generation MEBES e-beam writer reveals a maximum deviation of 2 pm from a nominal period of 1.070442 mum. (C) 2003 American Vacuum Society.