Measurement of displacement using phase shifted wedge plate lateral shearing interferometry

In present communication, a simple technique for measurement of displacement using phase shifted wedge plate lateral shearing interferometry is described. The light beam from laser is expanded and illuminates a wedge plate of relatively large angle. Light transmitted through the wedge plate is converged onto a reflecting specimen using a focusing lens. Back-reflected wavefront from the specimen is incident on the wedge plate. Because of the tilt and shear of the wavefront reflected from the wedge plate, typical straight line fringes appear. These fringes are superimposed onto a sinusoidal grating forming a moire pattern. The orientation of the moire fringes is a function of specimen displacement. Four step phase shifting test procedure has been incorporated by translating the grating in phase steps of pi/2. Necessary mathematical formulation to establish correlation between the 'difference phase' and the displacement of the specimen surface is undertaken. The technique is automatic and provides resolution and expanded uncertainty of 1 mu m and 0.246 mu m, respectively. Detailed uncertainty analysis is also reported. (C) 2015 Elsevier Ltd. All rights reserved.