Introduction

In the last chapter it has been shown how photographic processing may be used to form speckle pattern correlation fringes (Section 3.3). The resolution of the recording medium used for this technique need be only relatively low compared with that required for holography since it is only necessary that the speckle pattern be resolved, and not the very fine fringes formed by the interference of object and holographic reference beams. The minimum speckle size is typically in the range 5 to 100 μm (Section 3.1) so that a standard television camera may be used to record the pattern. Thus video processing may be used to generate correlation fringes equivalent to those obtained photographically. This method is known as Electronic Speckle Pattern Interferometry or ESPI and was first demonstrated by Butters and Leendertz (1). Similar work has since been described by Biedermann et al. (2) and Løkberg et al. (3,4). The major feature of ESPI is that it enables real-time correlation fringes to be displayed directly upon a television monitor without recourse to any form of photographic processing, plate relocation etc. This comparative ease of operation allows the technique or speckle pattern correlation interferometry to be extended to considerably more complex problems of shape measurement (Chapter 5) and deformation analysis (Chapter 7).

Intensity correlation in ESPI is observed by a process of video signal subtraction or addition. In the subtraction process, the television camera video signal corresponding to the interferometer image-plane speckle pattern of the undisplaced object is stored electronically.

In preparing this second edition the authors have taken the opportunity to modify Chapter 4 and Chapter 7. The purpose of this work has been to present the theory for the optimization of ESPI in a more general and exact form (Chapter 4) and to extend considerably the range of applications discussed (Chapter 7). The latter is representative of the wide range of problems to which holographic and speckle techniques are now applied. Chapter 7 also includes a brief review of techniques for automatic fringe interpretation.

Introduction: basic techniques and the general problem

It is the opinion of many scientists involved in the complex subject of surface deformation analysis that one of the most important techniques to emerge from the principle of holography (Section 1.7) is that of holographic interferometry. This enables the static and dynamic displacements of an optically rough surface to be measured interferometrically. First reports of the method appeared during the mid 1960s (for example, references 1–5) and were soon followed by numerous papers describing new general theories and applications (Section 2.9). One of the main reasons for such interest is that holographic interferometry clearly removes the most stringent limitation of classical interferometry (Section 1.5.5) i.e. that the object under investigation be optically smooth. Thus the advantages of interferometric measurement, for example, high sensitivity and non-contacting field view, can be extended to the investigation of numerous materials, components and systems previously outside the scope of optical study.

Let us first consider qualitatively how the holographic recording of a scattering surface can be used to detect the displacement of that surface. It has been shown in Section 1.7.3 that the developed hologram reconstructs a virtual image of the original object. If one views the precise superposition of the light from the reconstructed image and the real object through the hologram, then the interference of the two identical wavefronts results in a uniform field of view.

Introduction

Conventional shape measuring instruments use mechanical probes and give either point-by-point or line-scan information about shape (1). An optical method of measuring shape has the advantage of being non contacting and can also give a field view of the surface under investigation. Thus, there has been considerable effort directed towards the development of optical shape-measurement techniques.

Holographic methods of measuring surface shape are based on a two-wavelength technique first reported by Hildebrand and Haines (2). The two wavelengths can be produced by using two laser lines of different frequencies, or alternatively by altering the refractive index of the medium surrounding the object. The fringes represent the intersection of the object surface with a set of surfaces which in general are hyperboloids, but may be a set of equispaced planes in which case the fringes represent true depth contours. A new hologram must be made each time a new component is inspected.

ESPI can be used to compare the shape of test components with a master wavefront. The fringes obtained represent the difference in depth along the viewing direction between the master wavefront and the test component. The master wavefront may be produced by conventional optical components (i.e. flat, spherical or cylindrical) or may be generated holographically using a master component. The system enables components to be inspected in rapid succession.

When two beams of light which interfere to form a fringe pattern are projected onto the surface of an object, the form of fringes observed on the object surface depends on the shape of the surfaces (3).

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.