Defocus Measurement Using a Liquid Crystal Point Diffraction Interferometer

A point diffraction interferometer with a liquid crystal filter is used to measure a phase object. A reference beam is locally generated by a microsphere embedded within the liquid crystal layer. Phase shifts between the object and reference beams are introduced by varying a voltage across the birefringent nematic liquid crystal layer. Periodic phase measurement errors caused by modulations in the average intensity distributions are discussed, and a normalization method is described to reduce the errors. Experimental results are compared to a computer simulation to verify that the periodic errors are caused by intensity variations in the object beam.