From their first introduction, Incandescent light bulbs have been the standard lighting source since the early 20th century. Incremental improvements to the incandescent light bulbs improved their efficiency for light output vs power resulting in a top efficiency of about 17% for a 100 watt light bulb.
The next advance in lighting came with the introduction of the Compact Fluorescent Light bulb more commonly referred to as the CFL. A CFL is a fluorescent lamp designed to replace an incandescent lamp; some types fit into light fixtures formerly used for incandescent lamps. The lamps use a tube which is curved or folded to fit into the space of an incandescent bulb, and a compact electronic ballast in the base of the lamp. Compared to general-service incandescent lamps giving the same amount of visible light, CFLs use one-fifth to one-third the electric power, and last eight to fifteen times longer.
The next evolution of the light bulb is the LED light bulb or lamp. This is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. LED lamps offer long service life and high energy efficiency, but initial costs are higher than those of fluorescent and incandescent lamps. Chemical decomposition of LED chips reduces luminous flux over life cycle as with conventional lamps.
Commercial LED lighting products use semiconductor light-emitting diodes. LED lamps can be made interchangeable with other types of lamps. Assemblies of high power light-emitting diodes can be used to replace incandescent or fluorescent lamps. Some LED lamps are made with bases directly interchangeable with those of incandescent bulbs. Since the luminous efficacy (amount of visible light produced per unit of electrical power input) varies widely between LED and incandescent lamps, lamps are usefully marked with their lumen output to allow comparison with other types of lamps. LED lamps are sometimes marked to show the watt rating of an incandescent lamp with approximately the same lumen output, for consumer reference in purchasing a lamp that will provide a similar level of illumination. Efficiency of LED devices continues to improve, with some chips able to emit more than 100 lumens per watt.