Effect of acrylic cover on convergence property of aplanatic and hyperbolic Fresnel lenses

概要

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Underwater acoustic lenses are used as a beamformer for underwater imaging. However, lenses have aberrations, causing blur or distortion to acoustic images. A thin aspherical single lens is effective for aberration correction without attenuation or reflection of the sound wave. We have designed hyperbolic and aplanatic Fresnel lenses made of silicone rubber. However, the Fresnel lenses were too thin and soft to retain their shape. In this study, we designed acrylic covers to maintain the shape of Fresnel lenses, and we evaluated the effects of the covers on the convergence property by calculations and experiments. The covers were designed on the basis of wave theory. The optimal thickness was determined to be half of the wavelength in the cover. As a result, the covers did not effect the location of focal points. The covered aplanatic Fresnel lens showed an almost uniform gain in the range from 0 to 20°; however, the gain was lower than that of the uncovered lens. The covered hyperbolic Fresnel lens produced almost the same shape of the sound distribution as the uncovered hyperbolic Fresnel lens but with a lower gain than that of the uncovered lens at all incident angles. In conclusion, the covers decreased the lens gain but did not affect the angular properties. Thus, the covers are expected to contribute to practical applications using Fresnel lenses.