Distance measurement with extended range using lateral shearing interferometry and Fourier transform fringe analysis

We propose a distance measuring system based on lateral shearing interferometer (LSI) with extended range of measurement and improved resolution. The sensor is based on a wedge-shaped plate LSI and due to the presence of tilt, a finite number of fringes parallel to the direction of shear appear and hence significant spatial carrier frequency is generated at the focus position. On varying the distance of the object surface from the focus position, the orientation, fractional and integer order, and the width of the interference fringes change. It is shown that the orientation of the fringes is the most sensitive parameter, by means of which one can detect small changes in distance. The Fourier transform method for fringe analysis is used, and from the position of the maxima of the first-order Fourier spectrum the orientation of the fringes and hence the distance is measured. Experimental results of distance measurement by changing the focal length of the lens are presented. A large range of measurement without any fringe ambiguity problem can be achieved using the system.

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