Lens design and selection
For interior imaging, a lens with relatively small numerical aperture is required, in order to increase the working distance and limit spherical aberrations due to refraction at the surface of the sample. For surface imaging, the higher the numerical aperture the better. Rayleigh waves in the sample surface give vital information in acoustic microscopy of stiff materials, and the numerical aperture of the lens must be large enough to include the Rayleigh angle, which is around 30° for many materials. The strong refraction of acoustic waves passing from sapphire to water means that spherical aberrations at this interface are negligible even for a large numerical aperture single lens, but that an antireflection coating is necessary. The lens surface is typically ground into one end of a crystallographically oriented sapphire rod, at the Fresnel focus of a transducer at the other end. The spacing is also chosen to ensure that unwanted echoes arrive at different times from the reflection from the sample, which is used to form the acoustic image.
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