Penetrant inspection is a popular detection method because it is relatively inexpensive, can be done remotely, and can be administered on a surface that is oriented in nearly any direction. Penetrant Inspection may take the form of Fluorescent Penetrant Inspection (FPI) or visible Liquid Penetrant Inspection (LPI). Each has its advantages, so the selected method will be based on an internal or customer specification, or the type of part being examined. For example, FPI is often used on aircraft parts, while LPI is frequently used on machined surfaces, although neither alloy type nor product size limits which method is used.
In the early 1900s the railroad industry pioneered the oil and whiting penetrant inspection method using an oil solvent (the penetrant) and whiting or chalk coating (the developer) to detect imperfections. By the 1940s, fluorescent or visible dye was added and processes that included things like a defined soak/dwell time to generate uniform results were formalized.