Techniques of Displacement and Deformation Measurements in Speckle Metrology

The speckle effect has the rare distinction of being both a nuisance and an asset to the optics community working on the development of laser-based optical methods to monitor displacements and deformations of engineering structures. This chapter aims to review the speckle techniques applied to deformation analysis. The emphasis is to bring out clearly the limitations attached to these methods and propose conditions under which they can be used without introducing ambiguity into measurements. By far the simplest technique to set up and operate in speckle metrology is that of speckle photography. This method permits one to measure in-plane displacements, derivatives of displacements, and surface slopes, is easy to handle, and requires simple optical systems. The fringes in speckle interferometry, suitably interpreted as "speckle correlation fringes," arise from the correlation of two speckle pattern intensity distributions obtained before and after the object deformation.