This book covers a wide range of measurement techniques broadly referred to as Optical Metrology, with emphasis on their applications to nondestructive testing. If we look separately at each of the two terms making the generic name Optical Metrology, we find a link to two of the most distinctive aspects of humans: a particularly well developed sense of vision and a desire to classify things using numbers and rules.

Of all our five senses, vision is certainly the most developed and the closest to the rational part of our brain. It can be argued that our memory is strongly dependent on images and the brain is particularly good at processing the stimuli received from these images to extract information. Measuring, sizing and counting are, on the other hand, among the fundamental building blocks of modern society. The use of abstract quantities like size, value or intensity has simplified the description of complex enquiry and is the basis of modern science and economy. Hence, it would seem natural that the combination of two such basic aspects should result in the birth of a new field of science. However, it is known that his has not been the case. Optical Metrology remains classified as a group of special techniques used mainly in niche applications. Optical Metrology may be rightly described as an ensemble of techniques in which fields such as physics, electrical and mechanical engineering, and computer science merge and blend in new ways.

This book is intended as a tribute to the career of Professor Léopold Pflug. By looking back at his lifelong commitment to the application of optical metrology to the service of engineering sciences, more particularly devoted to the observation of the real behavior of structural components, one can retrace the major revolutions that have taken place in this domain. Starting his activity in 1971 as the head of the Laboratory for Stress Analysis at the EPFL in Switzerland, he first employed photoelasticity as a tool to improve the understanding of the real behavior of complex structures. However he soon recognized the necessity of working with the real materials used to build these structures instead of on replicas made of optically birefringent materials. He then focussed on the use of moiré techniques which sparked his fascination with laser-based holography and speckle-based methods. The advent of information technology led him to open up to the use of ESPI and digital image processing techniques. Finally, in the mid 1990s he became interested in the use of optical fibers as a tool for sensing deformations inside structures, not only on their surfaces as in the case of whole-field methods. It is interesting to note the parallel in the evolution of optical metrology vis à vis developments in other fields: the development of lasers led to holographic interferometry, the availability of frame-grabbers led to ESPI and the emergence of fiber optic communications opened the way to the development of fiber optic sensors. This puts in sharp perspective the strong dependence of optical metrology on the latest technology for its development. Also interesting to note is that all fields in optical metrology touched upon by Professor Pflug are still of great relevance, as shown by the contributions in this volume.

This book is, however, not intended as a commemoration, rather as an occasion to review the trends and undercurrents that are driving the field of optical metrology, with emphasis on nondestructive testing. All the authors were asked to summarize the recent achievements in their respective fields and to speculate about the future. As a result it has become apparent that it is difficult although not impossible to spot general trends in these disparate fields. Optical metrology has considerably benefited from some of the most important innovations of the recent past: lasers, computers and fiber optics communication, all of which found their direct inspiration from the developments in the world of electronics.

In recent years we have also witnessed a shift of power from states to corporations. This has created the need to produce quick results useful to industry. Optical nondestructive testing has certainly adapted to this evolution, and several contributions in this book show that the researchers in this field understand the importance of developing technology that can be used by the industry to solve specific problems. We should also not forget that optical nondestructive testing is essentially a "service technology" and should as such not only focus on serving its clients in the best possible way, but also should continually emphasize, extend and enhance its services to new users still unaware of its potential. Hopefully this book will help in spreading awareness of the potentials of optical metrology and in focusing on the challenges of the future.