There is disclosed an authentication device (10) for authenticating a luminescent security mark, the device comprising: an illumination source (30) configured to irradiate the security mark with a pulse of excitation radiation so as to cause the security mark to emit luminescent radiation that decays with time; a radiation detector configured to detect the luminescent radiation emitted by the security mark; and an optical waveguide (22) positioned relative to the illumination source (30) and the radiation detector and configured so as to guide by internal reflection both excitation radiation emitted from the illumination source towards the security mark, and luminescent radiation emitted by the security mark towards the radiation detector.