A lamp that creates radiant energy when its metallic filament is heated by an electric current. The filament is designed to produce radiant energy in the visible portion of the electromagnetic spectrum (light). The filament is of a special material that is supported in an envelope (bulb) that has been evacuated or filled with an inert gas such as argon, nitrogen, or krypton. In addition to light, the heated filament emits infrared and ultraviolet energy. When either of these radiations is accentuated, the lamp may be used as a source of that energy but with a reduction in luminous efficacy. The luminous efficacy (formerly called light-source efficiency) is the ratio of the units of light produced (lumens) to the power (watts) required, and is expressed in units of lumens per watt (lm/W).