White LEDs shining on blue paper. A light-emitting diode (LED) is based on a forward-biased heterojunction and emits light whose wavelength is determined by the bandgap of the semiconductor material. White LEDs are typically based on blue LEDs coated with a phosphor that converts part of the blue radiation to a broad spectrum centered in the yellow. The resulting spectral mix has appearance of white light to the human eye. White LEDs are beginning to be used for lighting and illumination applications. Compared to conventional light bulbs, LEDs are much more efficient, durable, and have longer lifetime.