What lies behind the widely used term LED?

This product is a light-emitting semiconductor: a Light Emitting Diode. The light is produced by electrons flowing between the anode and the cathode of the diode under the influence of an electric field, giving off energy as they move.

Products of this kind were already being manufactured at the end of the 1960s. Their light output — or, to use today's term, their luminous efficacy — was very low, so they could only be used as indicator lights.
Advances in semiconductor technology and in the chemical industry made it possible to manufacture diodes with far better luminous efficacy.

Today's technology allows the production of LED chips with a luminous efficacy of 150 lm/W or better.
LEDs are now the defining products of the lighting field, much as mercury and later metal halide and sodium lamps once were.
The energy efficiency of LED light sources has now reached 130–150 lm/W.

Compared with the light sources that LEDs were introduced to replace, this is a far more favourable figure. Considerably less energy is therefore needed to achieve the same lighting level with an LED light source — which also explains the success of their rapid spread.

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