Optical methods in non-destructive testing

There are a large number of methods that have been applied to optical non-destructive testing of objects. These can be grouped into two categories: point-wise and whole-field methods. Holographic interferometry and speckle methods due to their similarity are discussed in this paper. Further the emphasis is placed on methods that could be performed with electronic detection and hence are real-time in operation. Optical methods are non-invasive, non-contact and extremely sensitive. They can be grouped under two categories, namely point-wise and whole-field methods. The whole-field methods are often used for NDT. Information over the whole surface of the object is presented in whole-field methods. Holographic interferometry, speckle interferornetry, moiré methods and photo-elastic methods fall under whole-field methods. The first two provide an interferometric comparison and are subject matters of this paper.