Full-field imaging of guided ultrasonic wave fields using lensless digital holography

In this work, we present the full-field measurement of guided ultrasonic waves (GUWs) on technical surfaces using lensless digital holography. The setup works with a pulsed coherent laser illumination and a complementary metal-oxide semiconductor (CMOS) sensor without using any imaging optics. It provides highly resolved single-shot height measurements featuring the deformation introduced to the material by ultrasound excitation via piezoelectric actuators. For the evaluation of the resulting height distribution, we propose the multidirectional structure function offering pixel-precise determination of the wavelength. This ability is used to carry out a comparison between two aluminum plates of different thicknesses. Here, the resulting difference in wavelengths reflects the change in the thickness and is in agreement with simulated results for the two specimens, qualifying the proposed method as a detector for GUWs.