Liquid penetrant inspection is one of the oldest methods of non-destructive testing because of its simplicity and remarkable sensitivity to locating very fine surface discontinuities. Although it can only be used to inspect for flaws that break the surface of the sample, it can be used on ferrous, nonferrous, and even nonmetallic materials making it incredibly versatile. Low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending on the type of dye used.