1. A system for measuring the lateral displacement between edges located on two spaced apart objects comprising: means to illuminate said edges with collimated, monochromatic, spatially coherent radiation; means to form the focussed aerial image of a first edge which is located on one of said objects at the plane of a second edge which is located on the other of said objects so as to produce a diffraction pattern from the interference of the second edge and the aerial image of the first edge; means for sensing the light intensity of different points in said diffraction pattern and for determining the locations of the diffraction pattern minima, and; means for determining the lateral displacement between said first and second edges from the distance between the diffraction pattern minima.