Multi-wavenlengths Michelson interferometer based spatial phase shift shearography with color camera 2019-01-1269
The laser interferometric, optical full-field measurement method of digital shearography (DS) has already established for industrial NDT applications. The method with spatial phase shift (SPS), which can now be used qualitatively for the first time, allows the necessary phase information to be ob-tained in the video sequence, making dynamic investigations possible. SPS is linked to a reduced quality of the result, which is now largely compensated by the current state of camera technology. The simple, stable and cost-effective realization of SPS-DS using the carrier frequency method on the Michelson interferometer is particularly suitable for industrial use. The design can easily be ex-tended by a 4f system, allowing even large measuring areas to be inspected.
In addition, applications with multi-wavelength illumination were implemented with the aim of recording the entire three-dimensional object deformation in the video sequence. With SPS-DS this is typically achieved either by using several cameras or by using several carrier frequencies. Multiple cameras mean ever higher costs, an increased calibration effort and a higher susceptibility to errors. However, multiple carrier frequencies mean a reduction in the quality of the results.
The use of a Bayer Pattern color camera offers the decisive advantage here. Regarding the stable SPS-DS Michelson design, the paper presents the special settings compared to a monochrome camera, comparative considerations regarding the result quality, also with regard to a dual-shearing directions design, as well as the advantages of wavelength decoupling. The potential is shown by means of a test with the so-called 3 out-of-plane setup whereby the three-dimensional defor-mation is recorded with high quality in the video sequence.
Outlook considerations apply to the use of the described structure for NDT applications. Due to the increasing use of modern lightweight materials in the automotive industry, shearography is also becoming increasingly important in this industry sector. Especially methods with dynamic excitation, such as the lock-in method, are predestined for material analysis of composite materials.