Chapter 21: White light sources based on wavelength converters (click on figure for full-size image)

Fig. 21.1. White sources using phosphors that are optically excited by UV or blue LEDs. Fig. 21.2. Absorption and emission spectrum of a commercial phosphor (after Osram-Sylvania, 2000).
Fig. 21.3. Absorption and emission spectrum of the commercial dye “Coumarin 6”. The inset shows the chemical structure of the dye molecule. Fig. 21.4. Room-temperature bandgap energy versus lattice constant of common elemental and binary compound semiconductors.
Fig. 21.5. Emssion spectrum of Ce-doped yttrium aluminum garnet (YAG:Ce) phosphor for different chemical compositions. The excitation wavelength is 460 nm (after Nakamura and Fasol, 1997). Fig. 21.6. Chromaticity points of YAG:Ce phosphor, and the general area (shaded) accessible to white emitters consisting of a blue LED and YAG:Ce phosphor (adopted from Nakamura and Fasol, 1997). Also shown in the planckian locus with color temperatures.
Fig. 21.7. (a) Structure of white LED consisting of a GaInN blue LED chip and a phosphor encapsulating the die. (b) Wavelength-converting phosphorescence and blue luminescence (after Nakamura and Fasol, 1997). Fig. 21.8. Emission spectrum of a phophor-based white LED manufactured by Nichia Corporation (Anan, Tokushima, Japan).
Fig. 21.9. Chromaticity coordinates of a commercial phophor-based white LED manufactured in 2001 by Nichia Corporation (Anan, Tokushima, Japan). Also shown is the planckian locus and associated color temperatures. Fig. 21.10. Electroluminescence spectrum of conventional white LED and of high-color-rendering white LED. The high CRI results from the broader emission spectrum and the reduction of the notch in the spectrum (after Narukawa, 2004).
Fig. 21.11. (a) Conventional non-conformal and (b) conformal phosphor coating of semiconductor in white LED (adopted from Goetz, 2003). Fig. 21.12. Schematic structure of a photon-recycling semiconductor LED with one current-injected active region (Active region 1) and one optically excited active region (Active region 2) (after Guo et al., 1999).
Fig. 21.13. Photon-recycling semiconductor LED power budget with electrical input power P0 and optical output power P1 and P2. Fig. 21.14. Calculated power ratio between the two optical output powers P1 and P2 required to obtain white light emission (after Guo et al., 1999).
Fig. 21.15. Calculated luminous efficiency of a dichromatic PRS-LED versus its primary emission wavelength (after Guo et al., 1999). Fig. 21.16. Emission spectrum of dichromatic PRS-LED with current-injected GaInN blue LED primary source and AlGaInP photon recycling wafer (secondary source) emitting in the red (after Guo et al., 2000).