The afterglow is produced by the use of strontium aluminate. Strontium aluminate is a silver-coloured, odourless pigment that is insoluble in water. The crystal structure forms a three-dimensional network of corner-linked AlO4 tetrahedra, in whose cavities the strontium ions are located. Strontium aluminate is used as a luminescent pigment for photoluminescence and thermoluminescence. For applications as luminescent pigment, SrAl2O4 is doped with the rare earth elements europium and dysprosium. After activation, it glows for up to 10 hours. The doping components can also produce the colours yellow and blue. Before the development of this pigment in 1996, doped zinc sulphide was used, but its afterglow intensity is considerably weaker than strontium aluminate.