Phase images in non destructive evaluation

Optical image processing offers in many cases a reliable and very simple way to visualise phase images. The key concept of optical processing is that an expanded beam of coherent light reads the amplitude and phase of an image, and, when focused by an optical system, produces the bidimensional Fourier transform of the image, which is filtered by a suitable optical filter and reconstructed to give the processed image. Several methods are able to visualise phase images: those most known are the phase contrast, the strioscopy, the Schlieren and Shadowgraph techniques, and the technique of rotating derivative filters. An integrated interpretation of all the mentioned techniques basing on the general theory of optical processing of images constitutes an important theoretic result and is useful in the experimental practice, for instance in designing the optical system in relation to the characteristics and the desired aspect of the phase object to be visualised. From the visualised phase image is often possible to derive with a great precision the measure of characteristics of systems and materials and to analyse phenomena and processes. Optical processing is practically instantaneous, therefore it can follow very rapid processes; on the other hand it is essentially a planar technique, in the sense that only one plane is exactly focused, so that the examined images must be relatively thin.