Three-Band White LED Shows Stability and High Color Rendering Index

Anne L. Fischer

LED technology has advanced to the point where white LEDs are bright enough for general illumination, yet they are not without problems. Traditionally they have been developed by combining red, green and blue LEDs, but the problem with this approach is a varying rate of degradation, producing an unstable white light over a period of time.

Coordinates of the (a) blue, (b) green, (c) red and (d) three-band white LEDs under drive currents from 5 to 60 mA are plotted in the CIE 1931 chromaticity diagram.

The next step was to develop a blue LED precoated with a yellow phosphor that looks white to the human eye. This proved sufficient for LCD backlighting in displays, cell phones and traffic signals, but LEDs developed with this method lack sufficient red emission to produce a good color rendering index, which is of critical importance in applications such as museum gallery lighting or in medical procedures because it indicates how close the light is to sunlight (which is measured at 100). The higher the color rendering index, the more pure the colors appear.

A group of researchers at Everlight Electronics Co. Ltd. in Tuchen, at National Taiwan University and at National Taiwan Normal University, all in Taiwan, have been working with a three-band white LED. To begin, the group mixed the green and red phosphors into a powder and mixed that with silicone. The mixture was applied to the blue LED chip in a thickness that matched the size of the reflector cup. A transparent silicone was used to coat and protect the whole device.

In testing, the investigators found that increasing the drive current from 5 to 60 mA achieved a higher color temperature and color rendering index. At 20 mA, the luminous efficacy was about 30 lm/W, the color temperature was 5206 K, and the color rendering index was 90.5. When the drive current was increased to 60 mA, the chromaticity coordinates X and Y of the white LED as well as the temperature were found to be the same, but the color rendering index increased to 92.2. The scientists compared this with white LEDs developed using the more traditional method of mixing yellow and blue light, which results in a color rendering index of only about 80.

The next step will be to investigate higher-quantum-efficiency phosphors based on oxynitrides or nitrides to increase the efficacy of white LEDs.

Applied Physics Letters, March 19, 2007, 123503.