  Structural colors can be induced on metals not only to fabricate logos or
decorative motives but also to embed anti-counterfeit features for product
protection. In this study, stainless steel (EN 1.4301) plates are colorized by
growing a thin oxide layer using direct laser writing (DLW) and hidden
anti-counterfeit measures are included on their surfaces by direct laser
interference patterning (DLIP) processing. The periodic microstructures
resulting from the DLIP treatment have a spatial period of 1 um and act as
relief diffraction gratings, featuring a characteristic diffraction pattern.
These microstructures are not visible to the human eye but are easily
detectable upon shining a coherent beam on the surface. Furthermore, the
reflectance over the visible spectrum of the colorized surfaces with and
without the DLIP microtexture is measured, giving low differences in the color
perception following the so-called "CIE L*a*b*" color space. Finally, a
demonstrator is fabricated, in which colorized fields with and without the
security features are shown.
