The technique works because two different ions, gadolinium and europium, are in the phosphor material. The UV light boosts a gadolinium ion to its excited state. This excess energy is then transferred to two europium ions, both of which emit one photon of red visible light as they drop back down to the ground state. Meijerink and colleagues now hope to discover other phosphor compounds that can generate blue or green light so that a mixture of all three colours would produce white fluorescent light.