A modification of the reference surface can be applied. An on-axis polarization beam splitter, which is placed in the plane of the reference surface, separates the two surfaces, which are imaged onto the detector. Thus, true two beam interference can be obtained. The potential of phase shifting algorithms can be used. The interference contrast is high, even if large local variations of r(x,y) are present. In addition, high speed operation is enabled. The embodiment of a modified Fizeau type interferometer will be described.
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G. Fütterer, "Polarization-Fizeau interferometer enabling phase measurement with reduced uncertainty," Proc. SPIE 10326, Fourth European Seminar on Precision Optics Manufacturing, 103260B (15 June 2017);