Abstract
This paper describes a new type of underwater imaging system which displays an acoustic image formed by an acoustic lens in real time. The rod-like transducer constructed by arranging 500 narrow strips of a piezoelectric material is used in the system. This transducer with an acoustic lens radiates a fan-shaped sound beam. Scanning of the focused sound beam to illuminate some objects in water is performed by varying the sound frequency periodically. Measured values of this transmitting part are as follows: 1. Horizontal beam patterns of the transducer with a cylindrical lens having 10m focal length are measured. The beam width and focal depth are 6cm and 4m respectively (Fig. 7). 2. Direction of sound radiation varies at 30° with the change of frequency from 1. 0 to 1. 9MHz (Fig. 6). Back scattered sound waves from the objects are detected by a receiving equipment which consists of an aspherical solidliquid compound lens and piezoelectric receiving plate. Closely-spaced shallow cuts are made across the front face of the plate, to provide electrical and acoustical isolation between there adjacent line elements. The following are the characteristics of the receiving equipment: 3. Field of view of the compound lens is 20° with reduction of 3dB of the sound pressure (Fig. 15). 4. Effective value of the ratio of aperture-focal length of the compound lens is 1. 0. 5. Acoustical coupling of the adjacent line element of the receiver plate is estimated to be -8 dB (Fig. 15). Acoustic imaging at 10-m distance is achieved by supplying DC power of 30 watts into the driving amplifier, while the imaging speed is 37 frames per sec. The results of investigation show that this imaging system is available for the long-range and high-speed imaging in water.
