Introduction: a comparative summary of techniques

Two main techniques are grouped within the general classification of speckle pattern interferometry. These are:

1. (i) speckle pattern correlation interferometry; and

2. (ii) speckle pattern photography.

In both of these a fringe pattern is derived from an optically rough surface observed in its original and displaced positions. Depending upon the method of recording and fringe observation these fringe spacings can be made sensitive to the local displacements, displacement gradients (Sections 3.2 and 3.6) or the first derivative of the displacement gradient (Sections 3.3 and 3.7.2). As will become apparent, the directional and magnitude sensitivity of these fringes can also be varied over a substantially larger range than those in holographic interferometry. Furthermore the recording medium need not have such a high spatial resolution (for example Section 3.2.1). These factors combine to make speckle pattern interferometry a more flexible technique for displacement measurement than holographic interferometry despite the fact that fringe definition is usually poorer.

The first of these techniques, speckle pattern correlation interferometry, was described initially by Leendertz (1) and indeed it was the need to overcome some of the inherent problems of holographic interferometry (for example, Section 2.8.1) that stimulated the early work. A general interest in the properties of speckle patterns (Section 1.8) together with the work of Groh (2), (483–94) influenced the initial experiments. Groh had used the relocated negative of an image-plane speckle pattern as a shadow mask as a means of detecting fatigue cracks.