LED is an acronym for Light Emitting Diode Instead of a filament they use a semiconductor diode which emits narrow-spectrum light. Depending on the composition & condition of the semiconducting material used (Silicone, germanium), they come in either Infrared for sensing heat, Visible for every day use, or Near-Ultraviolet for spotting stains at a crime-scene. An LED consists of a chip of semiconducting material that has been “doped” with impurities in order to create a p-n junction. A p-n junction is basically a junction between an anode and a cathode. Current flows easily from the p-side to the n-side, but never in the reverse order. The wavelength and color of the LED depends on the band-gap energy of the materials forming the p-n junction.