Finding defects in materials using optics has a long and glorious history. The very first optical detection method used by humans was, of course, based on simple visual inspection. After the invention of the microscope (possibly the greatest optical invention ever), material inspection techniques took a great leap forward. In recent decades, with the invention of the laser and other advances in science and technology, there have been dramatic improvements in existing optical inspection techniques and the creation of many new ones. In this issue, we have collected four papers from different areas of optics that highlight important advances that have been made in material inspection techniques.
One of the most common consequences of reflecting laser light from a material surface is speckle. Originally regarded as a problem, speckle has since been utilized for the detection of various surface and subsurface defects, strains and deformations in many types of materials. The paper by Yoshida, et al., describes the exciting use of speckle interferometry to monitor the surface of a metal as defects coalesce to cause a fracture. This technique has important applications in understanding pre-fatigue stress in materials.
Ultrasound is the de facto standard for detecting defects deep inside many materials. In normal use, however, ultrasonic techniques require contact with the sample and, as such, are not always convenient to implement. Recently, new methods have been developed that use a laser to both generate and detect ultrasound. The paper by Blouin, et al., reviews laser-ultrasonic techniques and describes recent advances in the field.
Optical coherence tomography, an ultra-fast optical gating technique, has recently been developed for bio-medical applications (see, for example, Optics Express, Vol. 1, No. 13). It has been recognized that the gating properties of optical coherence tomography can be used to detect subsurface defects in certain translucent materials. The paper by Duncan, et al., surveys the application of optical coherence tomography to various non-biological materials.
The microscope is one of the oldest, most studied and most used optical tools. Yet, as shown in the paper by Andersen, et al., qualitative improvements are still possible. That paper shows how holographic techniques can be used to correct aberrations caused by a high numerical aperture Fresnel objective lens and other system components. The corrections result in high spatial resolution at a large working distance, allowing detailed inspection of materials in previously inaccessible environments such as vacuum chambers.
In conclusion, we hope that this small collection of papers gives a flavor of the new and exciting optical material inspection techniques being developed in laboratories worldwide.