Dye penetrant inspection (DPI), also known as liquid penetrant examination (LPE), is a type of non-destructive testing used generally in the detection of surface flaws in non-ferrous alloys. The dye penetrant inspection (DPI) method employs a penetrating liquid, applied to the surface of the component and enters the flaw, crack or seam. After the excess penetrant has been cleared from the surface, the penetrant is drawn back out and the crack is observed using a white light or UV light. Dye penetrant inspection (DPI) can also be used for the inspection of ferrous materials where magnetic particle inspection is difficult to apply. In some cases Dye penetrant inspection (DPI) can be used on non-metallic materials. Variations include the use of fluorescent dyes, where a black (UV) light is used to illuminate the residual penetrant. This Dye penetrant inspection (DPI) technique has even higher sensitivity than normal LPE but can only be used in the absence of other light sources.

Dye penetrant inpection can be applied to any non-porous clean material, metallic or non-metallic material, but is unsuitable for dirty or rough surfaces.