Interferometric shape measurement of rough surfaces at grazing incidence

Abstract

Interferometric shape measurements of technical objects are hampered by stochastic phase variations in the micro region caused by the surface roughness of the test object. Without special precautions, low-contrast interference patterns or even totally stochastic fringe patterns may be observed. However, illuminating the surface under test at grazing incidence reduces the phase variations because it increases the effective wavelength by a factor of 1/cos β, where β is the angle of incidence. Here, an interferometer for testing plane objects is described where the test wavefront is provided by means of diffractive optical elements (DOEs). A pair of identical DOEs is used to do the necessary beam splitting and beam shaping. The resulting fringes, showing the surface deviations of the test object from the ideal plane, are evaluated by means of phase-shifting interferometry. Serial measurements on different rough surfaces were made to find out the limits of grazing-incidence interferometry when technical objects are measured. To do this, different incidence angles have been realized by choosing a suitable set of diffractive elements. The contrast of the interference fringes has been taken as the evaluation criterion for the measurability of the shape of the rough surfaces within the grazing incidence interferometer.