Holographic and speckle interferometry, which are usually based on laser illumination, enable measurements of displacement (static or dynamic) and shape to be made on optically rough surfaces at sensitivities of the order of the wavelength of light. They can therefore be used to extend the methods of classical optical interferometry to the study of a wide range of objects and systems previously outside the scope of such interferometric investigation. The principle of holographic interferometry was established in the mid-1960s and is based on holographic wavefront reconstruction. Speckle interferometry developed from this work; it relies on the speckle effect which is a random interference pattern observed when coherent light is scattered from a rough surface. In both cases it was the development of lasers capable of generating visible radiation having both high coherence and intensity that enabled the methods to be applied to the solution of practical problems.

Although the techniques are relatively new, their application in such diverse areas as strain and vibration analysis, flow visualization, non-destructive testing and metrology has stimulated a large volume of fundamental and applied research; the results of this work are of considerable importance to a wide range of scientists and engineers. This book provides a self-contained description of the theoretical principles together with a detailed discussion of practical techniques and a survey of applications. The contents may be classified as follows:

Introduction

Introduction

The successful use of the techniques described in the previous chapters requires that some insight into experimental design and technique be gained. A good way of doing this is to actually carry out the required experiment together with the associated measurements. This approach is quite sound as long as it is borne in mind that a considerable amount of time and money can often be saved if the experiment is based on a brief theoretical study of the practical factors involved. The contents of this chapter are intended to provide the basis for such an approach. Readers may also find that the practical details discussed enhance their understanding of the theoretical principles already expounded.

Some factors affecting the selection of experimental technique

The techniques discussed in this book enable the following types of measurement to be made:

1. (i) Static and quasi-static surface displacements, using holographic interferometry (Chapter 2), or speckle pattern interferometry (Chapters 3 and 4).

2. (ii) Dynamic surface displacements using modified versions of the same general method as (i).

3. (iii) Surface shape based on dual wavelength Electronic Speckle Pattern Interferometry, dual wavelength holographic interferometry and fringe projection methods (Chapter 5).

Sensitivities for the above methods have been defined at various points in the text. The sensitivities of the various displacement techniques are summarized in Table 6.1 and the accompanying notes.

This information can be used as a guide when a technique is to be selected for the solution of a particular problem.

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.

Introduction

The purpose of this chapter is to provide the reader with an insight into the way in which the holographic and speckle techniques described in the previous chapters may be applied to the solution of practical problems. Over the past five years there has been a significant increase in both the scope and volume of applications (see, e.g. references 1 and 2). This has been due to a number of factors; these include: a greater familiarity with laser techniques, the availability of commercial equipment and, perhaps most important, the increasing requirement to improve the performance of engineering components, systems and materials in a wide range of critical areas. It is not, therefore, possible in a single chapter to provide an exhaustive description of all of the current application work. Instead, the contents have been arranged in sections considered to be representative of the most important areas of application. These are as follows:

1. (a) the study of the static and dynamic deformation of complex structures;

2. (b) non-destructive testing and material property investigations;

3. (c) component inspection (for example, profile measurement);

4. (d) the investigation of flow in transparent media.

A number of case studies within each of the above categories are described; each of these demonstrates how a specific technical problem has been solved. Sections 7.2 and 7.3 relate to (a), Section 7.4 relates to (b) while (c) and (d) are covered in Sections 7.5 and 7.6 respectively.