Dye Penetrant Inspection (DPI), also termed as Liquid Penetrant Inspection (LPI), is the portable and straightforward NDT method. It is an old method that started in the 19th century and is currently used to detect defects or flaws on any surface, such as cracks from fatigue, quenching, grinding, fractures, porosity, and incomplete fusion deficiencies in joints. The dye penetration principle uses capillary action; the liquid penetrant passes into the surface-breaking crack. The experts make dry powder application to draw out penetration in the gap, and thus, it produces a surface indication.

Visible 

Visible dye penetrant testing is when a white developer is used to draw the dye back to the surface from inside the discontinuities through capillary action. It is also known as the ‘colour contrast’ method. The colour contrast created is evident under natural light. 

Fluorescent 

Fluorescent dye penetrant testing uses a fluorescent dye that is typically green. It is for applying on the surface of a metal.  This process helps to check the defects so that integrity of the object stays safe. This method is not so expensive and is used widely in a variety of industries.