Random-phase-shift Fizeau interferometer

In the context of this article we demonstrate a novel Fizeau interferometric system that copes with the presence of vibrations. Besides the conventional high spatial, but low temporal resolution detector system (the CCD camera) used in phase shifting interferometry, an additional high temporal, but low spatial resolution detector system was integrated, in order to measure the random phase shifts that are induced under the influence of the vibrations. The additional sensor consists of three photodiodes. The acquired analog signals enable the measurement of the occurring phase shifts at three non-collinear locations on the test surface. Under the assumption of the rigid body shifts and tilts of the test object, the resulting phase shifts at the three individual locations enable the determination of the random phase shifts over the entire image aperture. While the random oscillations of the test object are continuously measured, the CCD camera acquires several interferograms. In consequence, a phase shifting algorithm for random phase shifts was applied. In order to prove the validity of the new interferometer, a test surface of known topography was measured. The results of the measurements in presence of vibrations show very good concordance with the surface data given by the supplier. The analysis of the root mean square (RMS) over ten different measurements shows a measurement repeatability of about 0.004 waves (approximately 2.5 nm for 632.8 nm laser wavelength).