Light-emitting diode:

One of the most common light sources used in electronics is the "Light Emitting Diode" (LED).  A LED is a two terminal semiconductor device comprising a p-n junction, which conducts in one direction only.  This semiconductor material emits light when the p-n junction is forward biased and a current is flowing through it.  LEDs can be manufactured to emit visible or invisible (infra-red) light.

Visible LEDs are often used as indicators in electronic equipment either singly, for indicating 'power on' for instance, or in arrays for alpha/numeric displays.

LEDs are reliable and have a very long life if treated carefully.  Light emission in different colours of the spectrum can, when required, be obtained by varying the proportions of the elements comprising the chip, and also by a technique of "doping" with other elements, i.e. nitrogen.  Current consumption (typically about 5 - 20 mA) generally limits the usefulness of a LED to equipment that is not battery powered.