Linear (E-scan) law count

A linear or E-scan keeps the angle fixed and moves the active group one or more elements at a time along the array. The number of positions is (N_total − n)/step + 1: the aperture has to fit inside the array, so the last group starts at element N_total − n + 1.

The index resolution of the resulting C-scan is the element step multiplied by the pitch, and the total electronic coverage is that scan length plus one aperture. A 64 element probe at 0.6 mm pitch using a 16 element aperture and a one element step gives 49 laws, a 0.6 mm resolution and 38.4 mm of coverage - the full length of the array.

Stepping two or four elements at a time cuts the law count and raises the frame rate, at the cost of index resolution. That is a sound trade for corrosion mapping where the beam is much wider than one pitch, and a poor trade for small defect detection where resolution is the point.

laws = (N_total − n) / step + 1
index resolution = step × p
coverage = (laws − 1) × step × p + n × p