White light is a combination of all wavelengths in the visible spectrum. Incandescent lamps inherently produce white light. LEDs do not. They emit light in a very narrow range of the spectrum, producing nearly monochromatic light—the color depending on the materials used to create the LED. “White light LEDs” are created in two different ways: phosphor conversion or RGB. In phosphor conversion, a blue LED is coated with a yellow phosphor, resulting in light which appears white to the eye. This method is lower in cost than the RGB approach. Phosphor converted chips are manufactured in large quantities in forms that are integrated into lighting fixtures.

In the RGB method, white light is produced by mixing the light from multiple red, green, and blue LEDs; sometimes amber is added to enhance the quality of the light. This results in great flexibility in the possible “shades” of white light produced, but is technically more demanding to manufacture, and thus more expensive at this juncture. RGB systems are generally found in custom-designed architectural lighting.

Light quality is indicated by two measurements, correlated color temperature (CCT) and the color rendering index (CRI).

CCT is that aspect of light that people refer to when they talk about “cold” fluorescent lighting; such lighting has a high CCT. CCT is measured in “Kelvins”; cool white light is 5000K while warm white light has a low CCT at about 2700K. Until recently, most white light LEDs had very high CCTs, often above 5000K, but warm white LEDs are now available. They are less efficient than cool white LEDs, but are comparitively efficient as CFLs.

The CRI is a measure of how color appears when illuminated by a light source, compared to reference sources such as incandescent light or daylight. A CRI of 100 is identical to the reference source, so the higher the CRI the better. Everyone has experienced the dull colors and washed-out faces resulting from old-style fluorescent tube lighting, which had a CRI ranging from 50 to 60. Phosphor-converted warm white LEDs are now being produced that are claimed to have a CRI of 80, a value most people find quite acceptable. Others exceeding 90 have also been reported.

The CRI, however, has been found to be inaccurate for white light RGB LEDs and there is controversy in the industry as to the reliability of the rating for other lighting types as well, so a new measurement system is under development.

LED technology is changing quickly; white light LEDs producing high-quality light will be commonplace in the next few years.

Source: UF/IFAS Energy Efficient Homes: Introduction to LED Lighting, Barbara Haldeman, Wendell A. Porter, Kathleen C. Ruppert, 2008.

Additional Resources

UF IFAS Energy Efficient Homes: Introduction to LED Lighting - http://edis.ifas.ufl.edu/FY1049

Using LEDs for General Illumination http://www1.eere.energy.gov/buildings/ssl/using_leds.html

Solid State Lighting Luminaires http://www.energystar.gov/index.cfm?c=new_specs.ssl_luminaires