It can be applied on all metallic or non-metallic materials.

It is usually a control method that should be done before applying another non-destructive testing method. It is the examination of the parameters affecting the quality such as discontinuity, structural defects, surface condition on the surface of a product, with or without the use of an optical aid (magnifying glass).

Depending on the accessibility of the examination surface, auxiliary devices such as endoscopes are used when necessary. The examination is made under sufficient light and with appropriate viewing angles.