1. A non-contacting method for measuring deviations between a reference wavefront representing a reference surface and a test wavefront representing a test surface comprising, establishing a white light interference pattern on a detector between two wavefronts, one wavefront from at least one point on a reference surface of known topography and another wavefront from a corresponding point on a test surface of unknown topography, repeatedly translating one of the wavefronts from said surfaces by known incremental distances relative to the detector from a starting position and establishing other white light interferences patterns, one for each translation, while establishing zero optical path differences between wavefronts from locations on the reference surface and corresponding locations on the test surface of unknown topography, reading light intensity levels at a number of sample points of the interference pattern for each translation, calculating a statistical function indicative of the contrast maximum for each of the sample points of the interference pattern over the range of wavefront translation, recording the positions of calculated maximum fringe contrast and the corresponding incremental distance of surface translation relative to the starting position for each sample point, whereby the points of maximum fringe contrast at a given step represent points, the locus of which defines contours for which there is zero optical path difference between the test surface and the reference surface.