The invention relates to a method for authenticating a security marking including at a long afterglow compound capable of emitting long afterglow luminescence light in a given wavelength region, the method allowing to detect a presence and an amount of said long afterglow compound based on comparison of a measured initial intensity value of a spectral component of the emitted light within said wavelength region and associated afterglow time with corresponding reference values representative of a reference amount value of a genuine long afterglow compound. The invention also relates to a reader and system operable to implement the steps of the method.