Structured light-based NDT techniques of interest in the aerospace industry

Abstract

Structured light techniques (Moire, projected fringes and other pattern projection techniques) have been used for many years in the aerospace and other industries for external defect location and characterization, 3D shape, deformation measurements and the like. The application of digital image processing methods to the images generated by these techniques has made possible the realization of fast and automatic systems for quantitative NDT measurements. This work is divided in two parts. First, the principal approaches to structured light-based NDT are reviewed, presenting the basis, performance characteristics as well as their expected ranges and resolutions. The second part concentrates on the digital image processing techniques of interest in the field of structured light-based NDT. As the images produced by these techniques are often in the form of phase-modulated fringe patterns, the branch of image processing that deal with this kind of images is denominated fringe pattern processing. Among the many existing fringe pattern processing techniques, the paper will concentrate on the phase-shifting methods (spatial and temporal) and the regularization and natural demodulation methods. It will be shown how when acquisition time is not critical, and several phase-shifted images can be acquired, phase-shifting techniques are well-suited for phase demodulation. Related with these techniques is the so-called phase unwrapping problem that will be discussed. Finally, fringe pattern processing techniques are discussed which can be applied in cases when acquisition time or the optical set-up only allows the use of one fringe pattern. In this case the problem of demodulating the phase is ill-posed from a mathematical point of view and special techniques as regularization and the so-called natural demodulation are needed.