<title>Real-time holography of ultrasonic surface waves</title>

Abstract

The holographic soundfield imaging technique combines the ultrasonic testing with the holographic interferometry. It makes possible the subsurface testing of components. The testing procedure results in an image that can be easily interpreted with regard to detected flaws. Ultrasonic surface waves are induced into the specimen and visualized by a real-time-image processing algorithm that subtracts the actual hologram from a hologram formerly stored without ultrasound. To get an image of the ultrasonic wave the intensity modulation of the illuminating laser beam is triggered by the ultrasound source. Former investigations of the holographic soundfield imaging produced satisfactory results concerning the lateral resolution for bonding flaws and cracks. Bonding flaws down to a diameter of 30% of the ultrasonic wavelength lying in depths down to 60% of the incoming ultrasound wavelength can be recognized. Recent development yields advantages with real-time holography mainly because of a shorter testing time. Furthermore, by this technique the processing of a single hologram allows the generation of several images with different ultrasonic frequencies and varying propagation directions. The sound wave is disturbed by the interaction between the ultrasonic wave and a flaw. Thus, with real-time holography the evaluation of components is possible during the propagation of the ultrasonic wave. Nowadays the real-time holography allows to perform two different irradiation directions with one hologram by using ultrasonic frequencies from 300 kHz up to 2 MHz. An ultrasonic amplitude of about 10 nm is required to attain a good contrast value for the visualized soundfield pattern.