The invention is directed to a method wherein at least a portion of a luminescent coating disposed on a surface of an article is exposed to ultraviolet light causing the luminescent coating to exhibit luminescence, detecting the luminescence, causing a comparison to be made between the luminescence and a predetermined standard, and classifying the article according to the comparison, wherein the luminescent coating has a luminescent chelate being a lanthanide and a ligand, the ligand being represented by the formula wherein R1 is alkyl, aryl, or heteroaryl; R2 is alkyl, aminoalkyl; aryl or heteroaryl.