<title>Interferometric testing of technical surfaces by grazing incidence interferometry in the infrared region</title>

Abstract

Increasing demands for accuracy in manufacturing and international standards of quality control require faster and more precise measurement techniques. Surface inspection and shape control of technical workpieces is commonly done by tactile profilometers. Interferometric testing of optically rough surfaces is faster, but the interference pattern is usually disturbed by high contrast speckle noise. Grazing incidence interferometry is an appropriate method to increase the effective illuminating wavelength. This leads to dramatically reduced speckle noise. Increasing of the wavelength from the visible to the infrared region is another opportunity to diminish speckle. An interferometric set-up combining both methods is presented. Well collimated laser light is splitted into several diffraction orders by a computer generated hologram (CGH). The zero diffraction order passes through to a second CGH and is used as the reference wavefront. The first diffraction order hits the object and is reflected to the second CGH where it is recombined with the reference wave. In the ideal case only uniform intensity is observed. Deviations from the ideal shape and misalignments of the object in the set-up lead to interference fringes after the second CGH. The fringe pattern is evaluated by using phase shifting interferometry. Further data processing eliminates the misalignment errors and reconstructs the shape of the object. The sensitivity of the interferometer depends on the design of the CGHs and can be adapted in a wide range to technical needs. The use of infrared light expands the measurement range. Rough surfaces can be tested with a convenient resolution in the direction of the optical axis. The capability of the IR-interferometer will be shown with some measurements of cylindrical workpieces.