How is white light made with LEDs?
Presently, there are two approaches to creating white light.
Mixed-color white light: One approach is to mix the light from several colored LEDs (Figure 4) to create a spectral power distribution that appears white. Similarly, so-called tri-phosphor fluorescent lamps use three phosphors, each emitting a relatively narrow spectrum of blue, green or red light upon receiving ultraviolet radiation from the mercury arc in the lamp tube. By locating red, green and blue LEDs adjacent to one another, and properly mixing the amount of their output (Zhao et al. 2002), the resulting light is white in appearance.
|Figure 4. Spectral power distributions of several types of LEDs.|
Phosphor-converted white light: Another approach to generating white light is by use of phosphors together with a short-wavelength LED. For example, when one phosphor material used in LEDs is illuminated by blue light, it emits yellow light having a fairly broad spectral power distribution. By incorporating the phosphor in the body of a blue LED with a peak wavelength around 450 to 470 nanometers, some of the blue light will be converted to yellow light by the phosphor. The remaining blue light, when mixed with the yellow light, results in white light. New phosphors are being developed to improve color rendering as shown in Figure 5.
|Figure 5. Spectral power distributions of early phosphor-based white LEDs (left), and white LEDs using more recently developed phosphors (right) with increased output between 600 and 650 nanometers.|