Speckle interferometry

This technique was invented in the 1970s to overcome the shortcomings of holography in the field of interferometry as regards the recording medium (generally argen-tic plates and films, then thermoplastic films). Unlike conventional holographic interferometry, speckle interferometry uses CCD cameras to calculate and visualize the displacement field of a scattering object. Speckle interferometry was first developed in the UK by A.E. Ennos, J.A. Leendertz, J.N. Butters, J.M. Burch and E. Arnold (see bibliographical references). It is ideally suited to industrial applications (more compact and easily transportable devices, lower costs, digital data processing in near-real time, etc.) and has therefore developed considerably in recent years, to the point where it is gradually replacing the holographic non-destructive testing systems used by large companies in particular, despite performance levels that fall far short of those of holographic interferometry, such as spatial resolution, the size of the object analyzed and the possibility of obtaining a 3D image of this object, for example.

CCD cameras have low resolution (6 µm at best today) compared with silver photographic plates (a fraction of a µm). The interference bangs created, as in holography, by the superposition of two mutually coherent waves must be readable by the electronic camera, which imposes an angle close to zero between the waves and an exploitation of the information that is different from holography. In holography, on the other hand, interference bangs recorded on photographic film or plates are used to reproduce the object wave analogically.

In both cases, recordings will be made for at least two different states of the object (at 2 different times), in order to measure the displacements undergone by the object between these two states. In speckle interferometry, the 2 fringe systems are used numerically to trace the displacements. In holographic interferometry, the restored waves corresponding to the 2 states of the object create a system of interference bangs on the 3D light image of the object, bangs which characterize the displacement field. It's these bangs that need to be exploited numerically to trace the displacements.

This article gives the basics of various speckle interferometry techniques currently used in industry, and shows just a few typical examples of applications.

This article is mainly based on reference [7]. For further information on speckle holography, please refer to references [2] to [6].

For all information on classical holographic interferometry, please refer to the article of this same section, reference [9], part one of this study on interferometric holography.