Development of light beam CT system for measurement of ultrasonic wave fields

When an ultrasonic wave is irradiated perpendicularly by a coherent light beam, the ultrasonic wave behaves as a phase diffraction grating, and the light is phase‐modulated. Since the degree of modulation of the light is proportional to the line integral value of the sound pressure in the light‐beam path, the sound field can be reconstructed by measuring it in various directions and by applying the CT method. This paper describes a development of the light beam CT system for measuring ultrasonic fields and the obtained results. A system to measure the projection data from various directions was developed, and sound fields produced by various transducers were reconstructed. The results were compared with the experimental data obtained by a hydrophone and the numerically calculated values. The degrading of the quantitativity of this method due to the fact that the ultrasonic wavefront is not really a plane (this is a crucial point in the method), was examined by using a computer simulation. The results show that the errors are negligible in the far‐field region of a plane‐type transducer or in the focal plane of a concave‐type transducer. However, the errors are significant in the rest of the region.