Dye Penetrant inspection (DPI), also called liquid penetrate inspection (LPI) or penetrant testing (PT), was one of the first techniques used to inspect for defects in materials. It is still widely used as a low-cost inspection method to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials and ferrous materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability.