There are two fluorescent phenomena, one is called auto-fluorescent (Phosphorescent), the other is called fluorescent. Auto-fluorescence is a very slow process, it can stay a very long time. Fluorescence is caused by energy emission from fluorophore which is a protein complex like a core surrounded by peptide (Fluorescent Proteins, e.g. Green Fluorescent Protien, Yellow Fluorescent Protein…). Fluorescense is stronger than autofluorescense, but the signal will quickly die after excitation…call Bleaching.