| 摘要 |
PURPOSE:To improve the resolution with a less attenuation by employing a piezo-electric element with a aperture larger than that of a lens with a length of a sonic propagation medium set at an odd fraction of the Fresnel focal distance. CONSTITUTION:A cylindrical crystal 150 employing a sapphire-fused quartz or the like as sonic propagation medium is cut off to such a length that the distance from the surface of a piezo-electric element 145 formed on the end thereof 150 to the back focal plane of a lens 155 formed on the other end thereof 150 is an odd fraction of the Fresnel focal distance po<2>lambda. The aperture of the piezo- electric element 145 is made larger than the aperture of the lens 155 enough to allow the passage of a main beam in the sound pressure distribution on the back focal plane. |
