A process for marking an article in a manner which is optically invisible and difficult to detect. The process is carried out by selecting a laser luminophore which fluoresces in a predetermined portion of the spectrum when exposed to an excitation light of predetermined wavelength and applying the laser luminophore to the article in an amount which is optically invisible when the article is exposed to electromagnetic radiation but which is sufficient for machine detection when the article is exposed to an excitation light of predetermined wavelength.