Measurement of small movements and vibrations by laser photography

Most of the holographic techniques applied so fa( for non·contacting measurements of surface displacements are highly sensitive. But the interpretation of the interference fringes obtained is usually rather difficult for a non-.expeft. especially for the study of tilts in the presence of lateral in-plane movements. Moire techniques could be applied to overcome some of the difficulties encountered but they require a relatively flat surface, which needs to be specially prepared. The application of speckling. a wel l-kno'Ml phenomenon in coherent optics, was found to be very convenient for the investigation of lateral in-plane displacements and vibrations in the presence of tilts. The speck le pattern can be regarded.as related to the object and hence wi ll be displaced if the object or part of it is mewed. For the analysis of in·plane movements or tilts the speckle pattern is photo~aphed in the image plane or the Fourier transform plane of the object, respectively. before and after the movement has taken place. From the diffraction effects of the recorded speckle patterns the movement of the object is determined. Different techniques to measure rigid'body translations, tilts and surface vibrations will be demonstrated and a comparison with other methods given together with the appropriate experimental results.