An integrated liquid crystal phase modulator for speckle shearing interferometry

A versatile phase-stepping speckle shearing interferometer is described that employed a calibrated liquid crystal as its phase modulator, operating at 514nm. By implementation of an algorithm upon phase-stepped shearographic images it was possible to create a phase map, that provided quantitative data regarding the derivative of any out-of-plane surface displacement. The use of a liquid crystal phase modulator in an optical arrangement had advantages over alternative methods. However, it is normally limited by spatial non-uniformity of the cells birefringence. The birefringence for a nematic liquid crystal cell was determined in several cross-sectional locations, before the cell was successfully integrated into a Michelson speckle shearing interferometer. Such a scheme offers flexibility in the shearing angle, and a measurement system that may operate over an extended range of monitoring wavelengths.