Shape and deformation measurement of 3D surface using phase stepping speckle interferometr

Simultaneous measurements of 3D surface shape and displacements using phase shifting digital speckle pattern interferometry (DSPI) have been studied. The criteria for the system have been investigated using in part computer simulated phase and speckle pattern data. A measurement system incorporating an optical arrangement of four beams and a personal computer based image processor has been devised and built. The facilities for the semiautomatic control of the interferometer and the acquisition, processing and presentation of shape, displacement, displacement derivative and strain data are presented. The shape and displacement data are acquired using the same optical system by using a unique procedure for the acquisition of 16 speckle patterns. Five of the speckle patterns are used to derive phase maps used for shape detection and 15 are used to derive phase maps used for displacement measurement. Shape is obtained through the lateral displacement of one of the illuminating beams. In addition to the lateral displacement, four phase shifts are required for the acquisition of data sufficient to calculate both shape and displacement for flat and curved surfaces. The combination of shape and displacement data allows displacement derivatives to be evaluated over the full field of view of the interferometer. A novel calibration for the phase shifting device, which allows the automatic, in-situ calibration and checking of electro-mechanical or electro-optic phase shift devices has been developed and incorporated into the measurement system. The two stage process uses a 'dark-frame' method to obtain the sensitivity and linearity of the device; after use the steps are checked using a digital correlation method. The correlation results are further used for the analysis of experimental uncertainties. When applied to the measurement of a cylindrical pressure vessel subject to an internal pressure the repeatability of the technique and image and data processing was of the order of 0.2 mm for shape and between two and ten microstrain for the displacement derivatives. Circumferential and shear strain results were found to differ at 95 per cent confidence level from conventional resistance strain gauge results.