Real-time speckle shearography system for defect detection in aircraft materials

A real-time speckle-based shearography system will be presented. It is a two camera system that records four phase stepped images simultaneously (pi/2 radians phase step), two per camera. This is achieved by the two Savart elements in the optical setup, that are used to create the proper phase steps between the two sheared wavefronts and to split the two mutually orthogonal components into two fully separated beams, thus creating two images, side by side on a single CCD camera. From those recorded images the phase can be computed using the four bucket algorithm. By measuring the phase before and after loading the sample under inspection, and subtracting the two phase distribution, information on the homogeneity of the sample is obtained. Loading is performed by heating the sample by use of a halogen lamp. Defects in the sample show up as irregularities in the phase difference distribution. The system has been used on parts of helicopters and airplanes, all within a laboratory environment. The use of the system in more hostile environments has been investigated and proofs to be much better than conventional, non real-time systems. Finally, a concept for an even more compact setup will be presented. In this setup only a single camera will be used. By employing the method of temporal phase unwrapping the phase front, and therewith the surface shape, can be monitored as a function of time.