Experimental Limitations of Phase Shifting Interferometry

Digital Phase Shifting Interferometry (PSI) is widely used in optical testing to determine the surface topography of a continuous surface. The height pro??le obtained using PSI is derived from intensity measurements made at different phases, which are seperated by a de??nite phase step. The unreliable piston travel of the peizo in response to a de??nite actuating signal is the main source of phase step and hence height errors in the height pro??le derived using PSI. The ??ve frame sequence suggested by Hariharan et. al. for intensity measurements makes PSI relatively insensitive to both positive and negative phase step errors (as large as 50% of the single desired phase step). However, we report on signi??cant errors in the PSI derived height pro??le that can result if a mathematically equivalent but experimentally different sequence than that suggested by Hariharan et. al. is employed for intensity measurements.