A phosphor is a substance exhibiting luminescence, i.e., emitting light (or other electromagnetic radiation) on nonthermal stimulation. There are two main types: the zinc phosphors, as used on cathode-ray tubes (where stimulation is by electrons); and the oxygen type, as coated on the inside of fluorescent light tubes to convert ultraviolet radiation to visible light. Zinc sulfide is often mixed with cadmium sulfide and a small quantity of metallic phosphates, silicates or fluorides to produce phosphors.