A type of lamp in which light is generated by fluorescence (see luminescence). A common form of fluorescent lamp consists of a gas-discharge tube containing a low-pressure gas, such as mercury, with the inner surface of the tube coated with a phosphor. When a current passes through the tube the ultraviolet radiation produced strikes the phosphor, which then emits visible radiation. Another type of lamp, the usual sodium vapour or mercury vapour street lamp, does not have a fluorescent coating; electrons in the discharge excite the atoms of vapour and these atoms fluoresce as they decay from the excited states thus produced.