The validity or authenticity of a latent illuminance material is established by heating or cooling the material to a predetermined temperature and then determining its decay time constant at that temperature. If the decay time constant is substantially equal to a predetermined value it is determined that the material is authentic. Thus, the authenticity of a material is determined by temporarily changing its decay time constant by way of temporarily changing the material's temperature using a heater or refrigerator. A light source illuminates the marker and the marker emits illuminance as phosphorescence. A photosensor detects the emitted illuminance, and the decay time constant is determined.