Optical methods in non-destructive testing

There are a large number of optical methods that have been applied to non-d estructive testing of objects. These can be grouped into two categories: po int-wise and whole-field methods. Holographic interferometry and speckle me thods due to their similarity are discussed in this paper. Further the emph asis is placed on methods that could be performed with electronic detection and hence are real-time operation. Optical methods are non-invasive, non-contact and extremely sensitive. They can be grouped under two categories, namely point-wise and whole-field met hods. The whole-field methods are often used for NDT. Information over the whole surface of the object is presented in whole-field methods. Holographi c interferometry, speckle interferometry, moire methods and photo-elastic m ethods fall under whole-field methods. The first two provide an interferome tric comparison and are subject matters of this paper.