A method for characterizing the performance of a lithographic imaging system has been devised which is independent of resist processing. A scanned grating technique has been adopted for measuring modulation as a function of both spatial frequency and defocus and it has been applied to the Markle-Dyson prototype (lambda=248 nm, NA=0.7) previously described. A theoretical model based on Hopkins’ formulation of partially coherent imaging was developed and showed agreement with measurement within experimental uncertainty. This demonstrates that the optics are indeed diffraction limited and are operating to specification.