Scan line count and index offsets
One pass of a phased array probe covers a band of material set by the electronic coverage width, or by the beam width where a single law is used. Covering a wider area means parallel passes, offset by less than the coverage so that adjacent passes overlap.
The index step is coverage × (1 − overlap) and the number of lines is ceil((required width − coverage) / index step) + 1. Overlap is not a luxury: encoder slip, probe wander and the tapering sensitivity at the edge of a beam all eat into the nominal coverage, and a nominally exact fit leaves unexamined strips.
Because the line count is a ceiling function, the last pass usually overlaps more than the others. That surplus is free coverage, and it is better placed at the end of the sequence than spread evenly, since it keeps the index positions on round numbers that are easy to set and easy to record.
Worked example
| Width to be covered | 300 mm |
| Coverage width of one pass | 40 mm |
| Overlap between passes | 20 % |
| Index offset between passes | 32 mm |
| Number of scan lines | 10 |
| Width actually covered | 328 mm |
| Coverage in excess of the requirement | 28 mm |
With 40 mm of coverage and 20 percent overlap the index step is 40 × 0.8 = 32.00 mm. After the first pass 300 − 40 = 260 mm remains, needing ceil(260/32) = 9 further passes, so 10 lines in all. They span 9 × 32 + 40 = 328.00 mm, 28.00 mm more than required.
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