A method for the detection of deviations or deformations of an object having a periodic structure

摘要

The object is illuminated by using two coherent beams of light. Two diffracted wave fronts with a different order of diffraction are selected from a number of wave fronts diffracted by the object. Both diffracted wave fronts are superimposed in order to produce an interference pattern. The two coherent beams of light are produced by splitting one coherent beam. At least one of both coherent beams of light is reflected twice before reaching the object, and the aggregate number of reflections of both coherent beams of light is an even number. The two beams illuminate the object at the same angle of incidence on either side of the normal.