Optical Testing of Technical Surfaces with Diffractive Optical Elements

Optical techniques which are well established for the testing of optical surfaces usually suffer from speckle noise caused by the roughness of technical surfaces. For this reason the shape control of technical workpieces is commonly carried out by tactile profilometers. An optical and much faster alternative to mechanical profilometry is grazing incidence interferometry. It suppresses speckle noise by increasing the effective test wavelength from λ to λ/cosϑ, where ϑ is the angle of incidence [1-3]. Diffractive optical elements, containing the shape information of an ideal object in their surface relief, are used as references for the workpiece enabling a null test of the entire mantle surface in a single step. The period p of the diffractive optical elements determines the diffraction angle α=arcsin(λ/p) and hereby the angle of incidence ϑ=π-α, the effective wavelength λeff=p and the sensitivity λeff/2 of the interferometer.