White LED spectrum.Colorless or white light as perceived by eye consists of many colors or wavelengths of light. Optical intensity as a function wavelength is called spectral distribution or spectrum. Shown here is the spectrum a white light-emitting diode (LED). The spectrum was obtained by spatially dispersing white light with a diffraction grating. A conventional LED is based on a forward-biased heterojunction and emits narrow-band radiation whose wavelength is determined by the bandgap of the semiconductor material. White LEDs are typically based on blue LEDs (InGaN, indium gallium nitride) that are coated with doped YAG (yttrium aluminum garnet) crystals. They act as a phosphor, down-converting 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. The price point for high-power white LEDs is approaching consumer levels. They are beginning to be used for lighting and illumination applications (called solid-state lighting). Compared to conventional light bulbs, LEDs are much more efficient, durable, and have a longer lifetime. Solid-state lighting is expected to eventually replace conventional incandescent and even fluorescent light bulbs. Photo May 2009.