Focal zone length

A focused beam is not sharp at one point and useless everywhere else. There is a band of depth either side of the focus over which the on-axis amplitude stays within 6 dB of the peak — the focal zone. Its length is FZ = N · S_F² · 2/(1 + 0.5·S_F), where N is the near-field length and S_F = F/N is the normalised focal length.

The focal zone is what actually defines the usable inspection band of a focused technique. Anything shallower or deeper than it is being inspected at reduced sensitivity, so a procedure that qualifies a focused probe at one depth cannot claim the same detection capability over the whole wall. For corrosion mapping and immersion scanning this is what sets how many focal depths (or focal laws) are needed to cover a thickness.

Because FZ scales with S_F², a shallow focus gives a very short, very sharp zone and a focus near the near-field limit gives a long, soft one. Focusing at S_F = 0.3 concentrates the energy tightly over a narrow band; focusing near S_F = 0.9 barely improves on the unfocused beam but covers a wide depth range. Choosing S_F is the real design decision — the calculator makes the trade-off visible.

The focal zone is not symmetrical about the focus in reality; it extends slightly further beyond the focus than in front of it. The start and end depths given here split the zone evenly about F, which is the usual working approximation.

N = D² / (4λ)
S_F = F / N
FZ = N · S_F² · [ 2 / (1 + 0.5·S_F) ]
Zone start = F − FZ/2,  Zone end = F + FZ/2