A LED (Light Emitting Diode) is a semiconductor diode that immediately emits light when electricity is passed through it. Over the past decade, this technology has been revolutionised. At the beginning of the nineties, LEDs were only available in red and green. It later became possible to produce blue LEDs and so ultimately to produce white light in one of two ways: either by combining red, blue and green (RGB), or by using a blue LED combined with a layer of white phosphorous.