The principle consists of wetting a clean test object with a fluorescent or colored liquid that penetrates into open discontinuities. After cleaning the excess penetrating liquid, a developer is applied to the surface and the capillary suction of the fluorescent or colored liquid remaining in the discontinuities gradually becomes visible.

With natural light or via a UV lamp (case of a fluorescent penetrant) one can then distinguish and quantify and/or measure the indications.

This method is relatively easy to implement and can focus on discontinuities in the order of µm (100 times finer than a hair).