Optical methods in experimental mechanics

Optical methods in experimental mechanics rely on several, and somewhat quite different, optical phenomena, which, over a time-scale of more than one century, have been conveniently exploited, in different ways, depending on the technology available at the time. But, despite the enormous technological progress in the last decades, and the fact that none of them (photoelasticity, moire´ , holography, speckle, etc.) is even close to phase out is a sufficient proof of the apparent invincibility and robustness of these methods.The huge impact of the introduction of computers interfaced with spatial light sensors (and, the next bigthing, with spatial light modulators) has made feasible the development of many innovative and complex optical measurement techniques which otherwise could not have even been conceived. However, since their operating principles are firmly rooted in the basic precepts of the optical phenomena, any real and measurable progress in optical methods is dependent on a deeper understanding of the phenomena underlying these techniques.Even if the impact of lasers and the possibility which have emerged through its invention cannot be understated, the developments that have ensued on managing light wavefronts has given us good insights on, for example, how to take advantage of a short temporal light coherence. Apart from commercially available OCT instrumentation in the medical sector, several applications of speckle interferometry and digital holography in experimental mechanics are reported, that utilize superluminescent diodes. White light photoelasticity has also been shown to be a practicable method for quantitative analyses.But at the basis of any real progress in a given scientific field, there is a widespread dialogue in the researchers’ community of its real scientific culture and nature. For example, although a researcher approaching speckle interferometry in the 1990s, could convince himself that the speckles needed to be resolved by a light sensor (by reasoning in accordance to the Whittaker-Shannon sampling theorem), practically no one nowadays would think of carrying out a measurement by speckle interferometry without integrating it at least over one speckle.In the paper written by the Guest Editors for the present Special Issue, apart from their personal experiences in the field of speckle interferometry, attention has also been paid to testify the special nature of the speckle and how special its intimate structure was, and which still largely remained to be explored. We cannot exclude that the speckle, an obnoxious noise in the past, but which also enjoys a strong reputation as information carrier, would not have more surprises in store for us in the future.