Phase-measuring algorithms to suppress spatially nonuniform phase modulation in a two-beam interferometer

Phase modulation of presently used phase-shifting interferometers is assume d to be spatially uniform across the observing aperture. However, calibrati on errors or the configuration of an interferometer can cause a spatial non uniformity in the phase modulation. Spatial nonuniformity causes a signific ant error in the measured phase when the phase modulator has nonlinear sens itivity. An even-order nonlinearity in the phase modulation in particular c ontributes to the errors. Lowest-order errors can be suppressed by adding a new symmetry to the sampling functions of the phase-shifting algorithm, ho wever the algorithm suffers from large random noise. The random noise is sh own to be decreased substantially by applying one more sampled frame to the algorithm. We derive new seven-sample and eight-sample algorithms that can compensate for a nonuniform phase shift and has much less random noise tha n the previous algorithm we proposed.