As first speaker of this meeting about the holographic non destructive testing, I think it's perhaps important to understand the new possibilities of the holography in the field of the non destructive testing to know before the progress of the other and sometimes the old non destructive methods as radiography and ultrasonic testing. The first question is : why it is necessary to make non destructive tests. We can separate two kinds of application field : First one : quality assurance before, during and after the fabrication of a piece or a construction. Second one : control in service during the use of this piece or construction. For the first field, we can see thanks the first figure that it exists an optimalization of the non destructive testing who gives the maximum benefit. To prepare the future an industry must assure the quality of his product a little beyond the maximum benefit.

There are many different non-destructive testing techniques : radiography, ultrasonics, magnetic particles, dye penetrant, eddy current, leak detection, acoustic emission, thermography, vibration analysis, strain gauges, etc... Some are of very large use, whilst other are quite specific. But each has its own range of sensitivity, and of capability to detect one or another kind of discontinuities or to measure one or the other characteristic of the material. So much that the different methods are actually complementary, and from a purely technical point of view, the possibility to compare the results of different methods applied to the same case is always useful!; but it is only feasible of the equipment keep such supplementary costs still acceptable. Basically, a sound knowledge of the capabilities and limits of the different NOT techniques is a basic step when an engineer has to assess the fi ability and acceptability of a part. The fabrication process determines which kind of discontinuity could occur during construction; and the type of equipment and its operating conditions determine what kind of flaws could occur during operation. It is possible then to choose the NOT method the best suitable for the case considered, and knowing the limits of that method, to decide how it should be completed by one or the other special and complementary technique, as NOT holography appears to be

This paper is an introduction to the Holographic Phenomena with a previous short re-call of the light properties necessary for this matter. Of course, this first section is without interest for specialists but can give some fundamentals in optics for the other readers. We have try to conserve a physical meaning in all the concepts described in this paper, hoping that this objective will be reached.

A great deal has been written about holography, especially in the years since Gabor won the Nobel Prize (1971) for his "invention and development" of the method. While it is fairly safe to state that the movie and T.V. industries are not on the verge of a revolution as a result of the highly touted three-dimensional characteristics of the process, it can be said that holography may offer considerable scientific potential in such diverse areas as computer storage, display systems, correlation techniques, medical diagnostics (acoustical holography) and radar (microwave holography), to mention just a few. Another promising application of holography, and one that has been given considerable attention at United Technologies Corporation and other industrial laboratories, is nondestructive testing. Consideration shall be given to this subject in the present paper by starting with a very brief review of holography (The Basic Tool), followed by a description of interferometric hologra-phy (Preparing the Tool for Use), and how it can be employed to nondestructively identify defects (Applying the Tool). This sets the stage for two final topics which establish the holographic process as a viable NDT technique: pulsed holography (Adapting the Tool to the Industrial Environment) and special HNDT techniques (Simplifying and Diversifying Tool Application).

Since the invention of interference holography in 1965, attempts have been made in laboratories throughout the world to adopt it as a practical tool for industry. The difficulties of doing this in a workshop environment are well-known, so that, as far as possible, studies of mechanical deformation and vibration have been carried out in the laboratory. Even so much ingenuity has to be exercised in the design of work holder and means of applying load or excitation to the component under investigation. At the National Physical Laboratory we have gained considerable experience in solving practical problems of this nature, often in the course of carrying out work under contract from a manufacturer. However, the ultimate aim of our work is to develop techniques that can be used directly in an indus-trial environment, preferably without the need to employ skilled scientific staff. Two approaches can be made to achieve this: 1) to develop a "conventional" holographic system tailored to the particular testing that is required, and to design it for easy and safe operation by NDT engineers. 2) to employ special holographic techniques that are relatively insensitive to the inevitable movements of the component under study that occur under workshop conditions. The latter can of course be achieved by employing pulsed lasers, but our work has been confined to the use of C.W. Laser systems, which 'are considerably easier and cheaper to implement.

Among the various methods presently used in the field of nondestructive testing, optical holography is expected to become a very useful and promising tool in the near future. In fact, holography offers a number of advantages which should be briefly outlined here : direct and overall visualization of defects (disbonding, formation of cracks, inhomogeneities...) on large sufaces (of several square meters). Furthermore there is no interaction with the object under test and the surface to be studied has not to be treated. Finally holography is characterized by a high spatial resolution and a great sensitivity (it is possible to detect deformations as small as a few microns). In contrast to other modern techniques,holography is relatively unexpensive and can be used on-site with pulsed lasers. The general principles of holography and of methods using holographic interferometry will be recalled (double-exposure holographic interferometry, real-time holographic interferometry, "time-average" holographic interferometry). Thereafter the activities in which ISL is presently engaged will be reported briefly, that is laboratory feasibility tests and experiments conducted on-site in an industrial environment with the aid, in general, of pulsed ruby lasers : testing of adhesive bonding in solid propellant rockers and in aircraft structures, detection and observation of cracking in fatigue tests, visua-lization of the modes of vibration of mechanical structures, experiments conducted on air-craft subjected to maintenance checking, etc.

Since 1969 interference holography has seen at the Fokker company a development from an exotic laboratory gimmic into a valuable method for quality control and evaluation of aerospace structures. It is used for inspection of adhesive bonded and advanced fibre composite structures as well as for evaluation of the deformation and resonance characteristics of structures of all kinds.

A very intersting optical technique which has considerable potential in the field of non-destructive testing is that of contouring. Contour generation means the formation of a fringes pattern giving the topographical map of three-dimensional object or of its vitual image.

AEROSPAT1ALE occupies a leading position in the European aerospace industry. Its industrial potential is exemplified by : - Its 4 divisions : Aircraft, Helicopters, Tactical Missiles and Space and Balistic Systems. - Its 11 factories. - Its 6 subsidiaries. The vitality of the firm can be demonstrated by a few figures : - Turnover (fiscal 1980 without the subsidiaries), 13, 169 millions French Francs. - Exports (in 1980), 48,2%. - Workforce (on 31st December 1980), 38,857 of whom 3,919 were with the subsidiaries. Among Aerospatiale products, we can quote : - Ariane. - Airbus. - Super Puma, Astar, Dauphin. - Tactical Missiles AS 15, AS 30, AM 59. - Satellites Meteostat, Intelsat V and Exosat. Certain projects were carried out in multi-national cooperation. These high-performance, high-reliability products presuppose the implementation of advanced technology. Hence, in order to maintain their standard, we use non-destructive testing thechniques such as X-rays or ultrasonics which have given complete satisfaction in the detection of flaws. However, to reduce the inspection contribution to the cost price of our products, we were led to develop new, large-scale methods, such as acoustic analysis and holographic interferometry. This paper covers the uses of holography in an industrial environment. We shall discuss the technical advantages of the method, illustrated by several examples,and the economical advantages, demonstrated by the practical example of an inspection line. Finally for the enhancement of Aerospatiale's technology (new materials, and holographic inspection) we will go on to outline other fields of activity.

Holographic interferograms are three-dimensional representations of a four-dimensional space. The fourth dimension is visualized by interference surfaces in space that inter-sect the three-dimensional image of the object and represents, e.g., loci of equal displacement. In sandwich hologram interferometry these interference surfaces can be titled by an analogous tilt of the sandwich hologram. Using this method of fringe manipulation makes possible the elimination of the influence of unwanted motions, the evaluation of the sign of object tilt, the direct study of the derivative of the fringe frequency, and the sutdy of both larger and smaller motions than possible using e.g., double-exposure holography. The tilt of the sandwich hologram causes a moire effect between the two sets of interference fringes that are recorded on the surfaces of the two hologram plates. The result is another moire effect in the image plane producing changes in the interference fringe pattern seen on the object. Thus similar results can be reached by producing a moire effect either in the Fourier plane or directly in the image plane.

A number of "aided visual" techniques are at the disposal of the control engineer when surface or near-surface cracks have to be detected. In increasing order of sensitivity and sophistication we can cite dye penetrants, magnetic inspection, specimen grid moire, speckle correlation, interferometry and holographic interferometry. Those methods have the common advantage to allow full-field inspection, as opposed to point-by-point techniques; a permanent record of the test can be obtained, allowing a posteriori inspection and/or follow-up of the defect.

A few contributions by Italian laboratories to non-destructive testing techniques pertaining to such diverse fields as shadow and projection moire, laser interferometry, moire holography are presented. The presentation includes a short introduction to the princi-ples, whilst emphasis is given to the applications.

An evaluative review of the literature of holographic NDE is summarized. Articles and reports are tabulated and categorized according to flaw type and techniques of object loading during holographic inspection. Technical and industrial status of the technique is evaluated and desirable research and development to enhance the future usefulness of holographic NDE are described.