Introduction

Among all possible applications of fringe pattern demodulation techniques, we have chosen to illustrate the case of speckle interferometry[14,15]. This method is extremely interesting to study the behavior of structures or mechanical assembly exposed to solicitations such as mechanical, pneumatic, thermal or acoustic loading.

The speckle phenomenon appears when a naturally rough surface is illuminated with spatially and temporally coherent light (usually from a laser).

The statistical properties of speckle usually depend on the coherence of the incident beam and on the statistical properties of the diffusing surface (or medium). In the case where speckle is produced with a highly coherent light illuminating a large diffusing surface, speckle statistics do not depend on the surface properties and we talk about “normal speckle pattern” . However, in general, speckle statistics depend both on the light coherence and the diffuser nature.

The principle of speckle interferometry is to register not only the speckle amplitude but also the phase variation before and after object distortion. The coding is based on the coherent superposition of a speckle pattern with a reference wave without speckle, or with another speckle wave coming from the same object or from another reference object.