Ferromagnetic (magnetizable) materials are also used to detect discontinuities on the surface and just below the surface. As the magnetic flux passes through the material surface, micron-sized iron powders are sprayed onto the surface. With the effect of magnetic polarization, iron powders are collected in the region of discontinuity.
It can be applied in two ways, Visible Light or Fluorescent, depending on the ambient brightness. Fluorescent gives more sensitive results than visible light.