Infographics / Ultrasound Physics

Beam shape: near (Fresnel) and far (Fraunhofer) fields

An unfocused beam from a transducer of diameter D. In the near field the beam is well collimated, roughly the transducer diameter, and narrows to its narrowest point at the end of the near field, whose length N is about D squared over 4 lambda. Beyond that, in the far field, the beam diverges with a half-angle whose sine is about 1.22 lambda over D. Increasing D or frequency lengthens the near field and reduces divergence.

Ultrasound beam: converging near field (Fresnel) and diverging far field (Fraunhofer) An unfocused beam from a transducer of diameter D. In the near field the beam is well collimated, roughly the transducer diameter, and narrows to its narrowest point at the end of the near field, whose length N is about D squared over 4 lambda. Beyond that, in the far field, the beam diverges with a half-angle whose sine is about 1.22 lambda over D. Increasing D or frequency lengthens the near field and reduces divergence. near field (Fresnel zone) well collimated, beam ≈ transducer diameter far field (Fraunhofer zone) diverges, half-angle sinθ ≈ 1.22 λ/D transducer D narrowest at end of near field θ near-field length N ≈ D²/(4λ) larger diameter D or higher frequency (shorter λ) → longer near field and less divergence
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