Measuring the modulus of the spatial coherence function using an error tolerant phase shifting algorithm

Phase shifting interferometry allows for the precise determination of the phase in interference patterns. However, the accuracy of the measured phase is hampered by unstable measuring conditions, one of the main contributors to phase dependent errors are reference phase mis-calibrations or mechanical instabilities. It has been shown that a special averaging technique allows the elimination of linear mis-calibrations, and in a more general sense, also the elimination of the linear contribution to the error curve for the reference phase shifter. We used a lateral shearing interferometer exploiting a diffractive grating wedge providing a linearly progressive shear for the measurement of a set of the intensities on the time axis for the determination of the modulus of the coherence function in order to avoid spatial neighborhood operations which means that we are able to measure the visibility pixelwise. It will be discussed how the measured visibility can be freed from phase dependent variations across the fringe field down to errors of second order and we will show how the a posteriori correction of periodic artefacts enables high quality modulus measurements.