A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p-n junction diode, which emits light when activated. When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the colour of the light (corresponding to the energy of the proton) is determined by the energy band gap of the semiconductor. An LED is often small in area and integrated optical components may be used to shape its radiation pattern. LEDs have many advantages including low energy consumption, long life span, improved physical robustness, small size and faster switching. Light-emitting diodes emit more lumens per watt than incandescent light bulbs. LEDs can light up very quickly, a typical red indicator LED will achieve full brightness in under a microsecond. The light-emitting diode can have a relatively long useful life.