The quarter wave plate is a birefringent optical device which divides an incident beam into two orthogonally polarized beams and introduces 90 degrees into one of two beams. Thus, one of two beams after the quarter wave plate has phase retardation of 90 degrees relative to the other beam. In optical system for speckle interferometry to measure in-plane displacement, two beams from a laser with vertical polarization are illuminated onto a specimen which will get through small displacement but the quarter wave plate is placed in optical path of only one beam. Interference fringe patterns of two beams reflected from the specimen are recorded by CCD camera. Theoretical analysis of the quarter wave plate with Jones matrix showed that the quarter wave plate could be used for phase shifting method in speckle interferometry.
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