White LED brought the real revolution in the lighting industry and was made possible with the creation of blue and UV LEDs. By nature of semiconductors, there is no unique band gap that can emit white light. In order to generate white light from LEDs two main processes are used in inorganic LEDs: wavelength mixing and wavelength conversion. White LEDs made from wavelength mixing are based on three colors; Red, Green and Blue are mixed. This technique presents the advantage of offering the control of the tone of the white light by changing the proportion of the different colors.
In the phosphor-based white LEDs the process is based on wavelength conversion due to photo luminescent properties of phosphor. In general white light is made from blue or near-ultraviolet LED whose radiation is absorbed by a phosphor-coated surface before being reemitted in a white color similar to fluorescent lights. The phosphor can be deposited on the LED die directly or on the plastic housing.
Generally, commercially available whit LEDs are based on InGaN blue LEDs coated with a cerium doped yttrium aluminum garnet phosphor. In order to use LEDs for general lighting and make them good replacements for incandescent and fluorescent lamps, the most important requirement that LEDs should fulfill besides efficiency and life span, and is a good color rendering. Good color rendering of LEDs should be combined with their good power efficiency. In the very first white LED, a 5 mm package, the phosphor dispersed within the epoxy surrounding the die or over the die itself. The color of LED lamps is completely controlled by the selection of the appropriate phosphors, giving the required control over the lamp color for many general illumination applications. However, achieving this goal will require optimization of all aspects of white LED lamps including the phosphor efficiency.
