| Making White Light with LEDs
White light can be achieved with LEDs in two main ways: 1) phosphor conversion, in which a blue or near-ultraviolet (UV) chip is coated with phosphor(s) to emit white light; and 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. Research continues to improve the efficiency and color quality of phosphor conversion.
The RGB approach produces white light by mixing the three primary colors - red, green, and blue. The color quality of the resulting light can be enhanced by the addition of amber to “fill in” the yellow region of the spectrum.
Comparison of White Light LED Technologies
Each approach to producing white light with LEDs (described above) has certain advantages and disadvantages. The key trade-offs are among color quality, light output, luminous efficacy, and cost. The technology is changing rapidly due to intensive private and publicly funded research and development efforts in the U.S., Europe, and Asia. The primary pros and cons of each approach at the current level of technology development are outlined below.
| |
Advantages |
Disadvantages |
| Phosphor conversion |
- Most mature technology
- High-volume manufacturing processes
- Relatively high luminous flux
- Relatively high efficacy
- Comparatively lower cost
|
- High CCT (cool/blue appearance)
- Warmer CCT may be less available or more expensive
- May have color variability in beam
|
| RGB |
- Color flexibility, both in multi-color displays and different shades of white
|
- Individual colored LEDs respond differently to drive current, operating temperature, dimming, and operating time
- Controls needed for color consistency add expense
- Often have low CRI score, in spite of good color rendering
|
 Most currently available white LED products are based on the blue LED + phosphor approach. A recent product (see photo) is based on violet LEDs with proprietary phosphors emphasizing color quality and consistency over time. Phosphor-converted chips are produced in large volumes and in various packages (light engines, arrays, etc.) that are integrated into lighting fixtures. RGB systems are more often custom designed for use in architectural settings.
|
