Currently, hydrophones are routinely used in ultrasound measurements in water. In spite of their high measurement accuracy and the possibility of measuring high frequency ultrasound (e.g., f = 20 MHz), hydrophones always suffer from the sound diffraction induced by the immersed hydrophone itself as well as the dependency of the output response on the angle of incidence. Since the discovery of acousto-optic interaction, optical techniques are expected to achieve accurate ultrasound measurements and avoid these two limitations, because light can retrieve information about ultrasound without material contact.
Light refractive tomography (LRT), which utilizes laser light to detect ultrasound pressure in fluids, has long been underestimated as a simplification of another optical technique, light diffraction tomography (LDT). After a thorough review of published research, it appears that LRT is so far the sole technique, which is noninvasive, omnidirectional and provides time-resolved results scaled in Pa. In the process of developing LRT into a reliable and useful ultrasound measurement technique, achievements have been made in four different aspects:
- Accuracy: With the aid of numerical simulations, potential systematic errors are quantitatively analyzed. The subsequent development of simple and effective methods ensures that critical error sources have small influences on results (typically 1MPa) arising from an ultrasound device driven by compressed air. Finally, the conclusion is drawn and some further research possibilities are suggested.