Electric Lighting

Electric lighting is the use of electrical energy to produce visible light for illumination. The earliest practical electric lamp was the incandescent light bulb, in which current passes through a thin tungsten filament inside a sealed glass envelope. The filament resists the current and becomes hot enough to glow, emitting light along with a large amount of heat. The bulb is filled with an inert gas or evacuated to slow the evaporation of the filament and extend its life. Incandescent lamps are simple and give pleasant warm light, but they convert only a small fraction of their energy into light, which makes them inefficient.

Many other lamp technologies improve on this efficiency. Fluorescent tubes pass current through a low-pressure gas to produce ultraviolet light, which a phosphor coating converts to visible light. Discharge lamps such as sodium and metal-halide types are used for streets and large spaces. Light-emitting diodes (LEDs) generate light directly in a semiconductor junction and have become the dominant technology because they are efficient, long-lived and compact. Each type differs in colour rendering, lifespan and the way it must be controlled.

Powering these lamps requires appropriate wiring. Lighting circuits are usually separate from socket circuits and use conductors sized for the smaller currents that lamps draw. Switches interrupt the line conductor to control lamps, and arrangements such as two-way and intermediate switching allow a single light to be controlled from several points, as on a staircase. Many lamps need control gear: fluorescent and discharge lamps use ballasts to limit current, and LED fittings include drivers that supply a regulated current. Correct earthing of metal fittings and the use of protective devices keep lighting installations safe.

Frequently asked questions

Why are incandescent bulbs considered inefficient?

Most of the electrical energy heating the filament is released as heat rather than visible light, so only a small fraction becomes useful illumination.

What is two-way switching?

A wiring arrangement that lets the same light be turned on or off from two different switch positions, commonly used at the top and bottom of stairs.

Why do fluorescent and LED lamps need extra control gear?

Fluorescent and discharge lamps need a ballast to limit current, while LEDs need a driver to supply a steady regulated current, because neither can be connected directly to the mains like a filament bulb.