Zone-plate Lenses for Sound Waves in Water
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A question that naturally arises in a discussion of acoustical imaging is whether one should use lenses or phased acoustical arrays. Although phased arrays offer more flexible solutions to some problems, there are several reasons why lenses may be preferable in many situations. The use of a phased array requires sophisticated signal processing to provide an image, and, since the image is usually built up point by point by focusing sound on one object point at a time, the imaging process is slow at long working distances. A zone-plate lens for sound waves has been constructed that gives an optimal image of a point on the axis. Following Stamnes et al (1982a) we compare the image fields produced by the zone-plate lens and the corresponding Fresnel zone plate. The focal shift is clearly demonstrated, in which the intensity distributions along the axis are displayed for the Fresnel zone plate and for the zone-plate lens.