A tagging system contains first taggants and second taggants. The taggants differ from each other so that the first taggants have a first emission spectrum and a first absorption edge and the second taggants have a second emission spectrum and a second absorption edge. In the tagging system, the first taggants are arranged to be within an interaction distance of the second taggants so that energy transfer between the first taggants and the second taggants causes an emission spectrum of the tagging system to differ from a sum of the first emission spectrum and the second emission spectrum.