White-light sources based on wavelength converters
The generation of white light by a semiconductor LED whose light is partially or fully used to optically excite one or several phosphors, is a viable and common method to generate white light for general illumination applications. There are several different approaches to generate white light based on phosphors excited by semiconductor LEDs, which are shown in Fig. 21.1. They can be classified in dichromatic, trichromatic, and tetrachromatic approaches. These approaches use either UV-excitation sources or visible-spectrum-excitation sources (mostly blue semiconductor LEDs).
Generally, the luminous source efficiency decreases with increasing multi-chromaticity of the source. Thus, dichromatic sources have the highest luminous efficacy of radiation and also the highest potential luminous source efficiency. On the other hand, the color-rendering capability is lowest for dichromatic sources and it increases with the multi-chromaticity of the source. The color-rendering index can reach values very close to CRI = 100 for tetrachromatic sources.