IESNA Definition: a generic term for a source created to produce optical radiation. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible.

Three broad categories of electric lamps are:

• incandescent - A filament, usually of coiled tungsten wire, is heated to incandescence by the flow of current.
• fluorescent - Low pressure mercury is ionized inside the lamp, producing primarily ultraviolet radiant energy which causes phosphors to fluoresce.
• high intensity discharge - Pressurized gases inside an arc tube are ionized by current flow between electrodes, emitting light. For each of the three types of HID sources, the gases in the arc tube are different:
• mercury vapor -mercury, plus small quantities of argon, neon, and krypton
• metal halide -same as vapor lamps, plus iodides of sodium and scandium; other metal halide lamps may have iodides of rare-earth elements.
• high pressure sodium -sodium, mercury amalgam, and xenon