Dye Penetration Testing otherwise known as Liquid Penetrant Testing or Colour Check Testing and is most widely used as an NDT method for surface testing of material. It can also be used as a medium to check for leaks as well in various items.

The test area is coated with a visible or fluorescent dye solution and allowed to soak and dwell on the surface for around 15 mins. The excess dye is removed from the surface, and then the developer applied. The developer acts like blotter and will draw the penetrant out from any imperfections that are open and exposed to the surface. Visible dye and the vivid colour contrast between the penetrant and developer makes the bleed-out process easy to view by the technician performing the test. With fluorescent dyes and an ultraviolet lamp the bleed-out becomes fluoresce bright, thus allowing the imperfection to be seen readily in the dark.