Liquid Penetrant Testing (PT) is an nondestructive inspection method that is used to reveal surface breaking flaws by the bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence.
Colored penetrants (contrast) require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".