Notes on the application of Electronic Speckle Pattern Interferometry

The application of ESPI for 3D deformation measurement in material testing, dimensioning of structures and quality control, requires a stable and compact interferometric device. This device can be realized by optoelectronic components as laser diodes, fiber optics and CCD cameras. The developed interferometer is not only suitable for analysis of static problems, but also for measurements of vibration behavior of objects in real-time, by intensity modulation of the illuminating laser light. Disturbing influences of oscillating rigid body movements between ESPI and the considered object can be eliminated by a Michelson interferometer, which is adapted to the ESPI. By applying phase shifting techniques to the recorded speckle patterns, results of high accuracy can be achieved. The image processing of the phase maps is performed by unwrapping algorithms, which consider discontinuities as unsteady deformations, noise and inexactly modulated areas. For a practical handling of the data, a comparison with numerical calculation, the interferometric data are transformed into the meshes of an FEM (Finite Element Model). This procedure, which consists in transforming the measured 3D displacement vectors into nodes of FE meshes, can be performed by orientation and calibration of the ESPI. For a correct interpretation of the ESPI fringe images the contour of the considered object surface has to be integrated into the analysis process. The contour can be determined automatically by a topometric system based on the evaluation of projected fringes by a combined grey code and phase shift method.