Some liquids are able to penetrate surface defects or very thin discontinuities by capillarity and not by gravity. This kind of inspection is especially suitable to highlight and identify surface discontinuities, such as cracks, porosities, and laps in a quick and affordable way and great accuracy on any type of material.
Depending upon the technique applied, the lack can be identified in different ways: contrast of colours or by the technique of fluorescent liquid, easily detectable in the dark (Woods lamp)
