Application of Fast Fourier Transform (FFT) in Laser Speckle Image Pattern Correlation technique for the metrological measurement

This paper describes Laser Speckle Image Pattern Correlation (LSIPC) technique, a nondestructive method for the measurement of deformation/displacement of the specimen in metrology using Fast Fourier Transform. A diffuse object is illuminated with laser light, a random interference effect known as `speckle pattern' is produced. If there is movement in the object, the speckle patterns fluctuate in intensity. These fluctuations can be used to provide information about the movement. The experimental arrangement is based on the Michelson Interferometer method. The object is a thin steel plate fastened by means of screws at four points. A screw set into the center of the plate and inducing flexure of the plate is used to produce deformation of the plate. Illumination of the plate and photography were performed along directions close to the normal. The principle of this technique is capturing of two speckle patterns of a specimen, one before deformation (Reference image) and the other after deformation/displacement (Deformed image) by a CCD camera and registered by a frame grabber. The above deterministic increment in the phase difference of the two interfering fields is displayed in the difference pattern of intensities. Small sub images are obtained by segmenting process and analyzed point wise. An algorithm has been written using Fast Fourier Transform to obtain the information about the deformation. This method consists of simple and robust optical setup to perform non invasive measurement to the sub micron level for the measurement of deformation.