The possibility of employing a digital speckle pattern interferometer with optical fibers for nondestructive testing of artwork has been evaluated. A simple and accurate fiber-optic interferometer, able to perform real-time measurements in a hostile environment, has been realized with a minimum of optical components and a minimum of adjustments. As a result a deformation map of wooden panel paintings or mural frescoes under thermal stress may be depicted. Experimental results, obtained by the application of this interferometer both to laboratory models and to mural frescoes in situ, are presented.

The maps of phase derivatives are extracted here by direct manipulation of phase-shifted interferograms. There are three main advantages: There is no need for prior phase evaluation or unwrapping procedures, and only a short processing time is needed. By digital integration of the derivatives the absolute phase map can also be retrieved without unwrapping procedures. A general description of the method is presented and discussed. For example, the proposed technique has been applied to the study of the deformation of a test object by the manipulation of four phase-shifted interferograms.

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