Chapter 20: White-light sources based on LEDs (click on figure for full-size image)

	Fig. 20.1. LED-based approaches for white sources including single-chip and multiple-chip, di-chromatic, tri-chroamtic, and tetra-chromatic approaches.		Fig. 20.2. Monochromatic complementary wavelengths resulting in the perception of white light at a certain power ratio (after Wyszecki and Stiles, 1982).
	Fig. 20.3. Calculated luminous efficacy of dichromatic white light source (with chromaticity point at D65 standard illuminant) for different linewidths Delta E as a function of the primary wavelength. Also shown is the complementary secondary wavelength (after Li et al., 2003).		Fig. 20.4. Structure of a monolithic dichromatic LED with two active regions (after Li et al., 2003).
	Fig. 20.5. Room temperature (a) photoluminescence and (b) electroluminescence spectra of monolithic dichromatic LED with two active regions (after Li et al., 2003).		Fig. 20.6. (a) Emission spectrum of tri-chromatic white multi-LED source with color temperature of 6500 K (solid line) and gaussian fit (dashed line). The source has a luminous efficacy of radiation of 319 lm/W and a color rendering index of 84. (b) Photograph of source assembled of 5mm LEDs (after Chhajed et al., 2004).
	Fig. 20.7. Contour plot of luminous efficacy of radiation and CIE color-rendering index of white trichromatic LED source with color temperature 6500 K as a function of the three wavelengths for a linewidth (FWHM) of 5 kT (after Chhajed et al., 2004).		Fig. 20.8. Fig. 20.8. Contour plot of luminous efficacy of radiation and CIE color-rendering index of white trichromatic LED source with color temperature 6500 K as a function of the three wavelengths for a linewidth (FWHM) of 8 kT (after Chhajed et al., 2004).
	Fig. 20.9. Emission spectrum of trichromatic white LED source for different ambient temperatures (junction heating neglected). Optical power, linewidth, and peak wavelength change with temperature. As a result of these changes, the color temperature of the source increases (after Chhajed et al., 2004).		Fig. 20.10. Change in chromaticity of trichromatic white LED-based source. The source color temperature is 6500 K when devices are at room temperature (after Chhajed et al., 2004).
	Fig. 20.11. Change in (a) x, y and (b) u', v' chromaticity of trichromatic white LED source. Tc = 6500 K when p-n junctions are at room temperature (after Chhajed et al., 2004).