Colour Quality of White LEDs

Colour quality is one of the key challenges facing light-emitting diodes (LEDs) as a general light source. This paper reviews the basics regarding light and color and summarizes the most important color issues related to white light LEDs.

Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. Instead, LEDs emit light in a very narrow range of wavelengths in the visible spectrum, resulting in nearly monochromatic light. This is why LEDs are so efficient for colored light applications such as traffic lights and exit signs. However, to be used as a general light source, white light is needed. The potential of LED technology to produce high-quality white light with unprecedented energy efficiency is the impetus for the intense level of research and development currently being supported by the U.S. Department of Energy.

White Light from LEDs

White light can be achieved with LEDs in two main ways:
1) phosphor conversion, in which a blue or ultraviolet (UV) chip is coated with phosphor(s) to emit white light;
2) RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light.

The phosphor conversion approach is most commonly based on a blue LED. When combined with a yellow phosphor (usually cerium-doped yttrium aluminum garnet or YAG:Ce), the light will appear white to the human eye. A more recently developed approach uses an LED emitting in the near-UV region of the spectrum to excite multi-chromatic phosphors to generate white light.

The RGB approach produces white light by mixing the three primary colors red, green, and blue. Color quality of the resulting light can be enhanced by the addition of amber to “fill in” the yellow region of the spectrum. Status, benefits and trade-offs of each approach are explored here after.

What is White Light?
What appears to our eyes as “white” is actually a mix of different wavelengths in the visible portion of the electromagnetic spectrum. Electromagnetic radiation in wavelengths from about 380 to 770 nanometers is visible to the human eye.