WHAT IS A LED? The LED diode (Lighting-Emitting Diode), appeared back in the early 60, is a semiconductor device which if it is crossed by a current through the PN junction emits photons. A LED transforms the electric energy into a luminous energy. The color (wavelength) of the emitted light, the efficiency of the electro-optic conversion and so the obtained luminous intensity depend on the nature and condition of semiconductor which is used and it extends from the infrared, to the visible light gamma to the proximity of ultraviolet. LED are reversible devices: in effect, if the junction is directly exposed to solar rays or to a strong luminous source on the reo-hole (REOFORI) is possible to notice a tension.