In order to avoid frequency aliasing,improve the spatial resolution of the phase map in Fourier transform Profilometry(FTP),an approach based on the digital time-multiplexing technique is proposed to remove the background component from the deformed fringe pattern. Firstly, a sinusoidal fringe pattern is projected onto the tested object by digital-light-processing( DLP) projector, the fringe pattern modulated by the object’s surface is captured by a CCD camera.Secondly, apply Fourier transform to the captured fringe pattern to obtain the spectrum. Thirdly, rotate the specimen’s fringe pattern 90-deg to obtain the rotated fringe pattern , then obtain the new spectrum corresponding to the rotated fringe pattern.Fourthly,the new spectrum is subtracted from the original spectrum ,clip the negative going values in the resultant spectrum by digital manipulation.Fifthly, filter out the first-order spectrum from the resultant spectrum by the band filter,apply inverse Fourier transform to the selected spectrum to obtain complex fields,then retrieve the phase, unwrap the wrapped phase map by the phase unwrapping algorithm.Finally, the simulation and experimental evaluations are conducted to prove the validity and performance of the proposed method. The results are analyzed and compared with those of the conventional method.The effectiveness and superiority of the proposed method have been demonstrated and verified.
The Moiré method is a well-known technique which can be used to measure in-plane and out-of-plane deformations. The basis of all moiré methods is gratings, which for metrological applications are primarily an array of dark and bright lines or a set of crossed array of lines or dots. Moiré is a process where the intensity distributions of two dissimilar grid patterns are combined, for example by superimposition. The direct approach to generating moiré patterns is utilizing the aliasing phenomenon, which is generally avoided in traditional image processing, now can be used to analyze the deformed specimen grating for deformation studies. An alternate approach, the digital moiré method, is to use logical operations for generating moiré patterns. Accepted as a novel mathematical tool, wavelet transform has a great advantage for image processing. In this pager, we apply it to simulate the aliasing phenomenon. In analysis of moiré fringes, phase shifting is by far the most powerful of fringe analysis techniques. For digital moiré, phase shifting can be achieved simply by translating the software generated gratings by the appropriate fractional steps. An application for large deformation of rubber material is given; the result shows that the method is feasible.