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6 Handbook on the Physics and Chemistry of Rare Earths white light is obtained using an UV LED to excite red–green–blue (RGB) phos- phors (b), using a blue LED to excite red–green (RG) phosphors (c), and by mixing reds, greens, and blues, ie, RGB, LEDs (d). Type (a) is now most com- monly used with InGaN LED chips and yellow YAG:Ce3+ phosphors because of easy fabrication and low cost. However, the white light obtained by this device has some problems such as poor color rendering index (CRI) and low stability of color temperature (Sheu et al., 2003; Steigerwald et al., 2002). Type (d) has the advantage to be able to create not only white colors but also various colors, although electronic tuning is complicated to require different driving currents for different color LEDs (Shur and Zukauskas, 2005). Type (b) has similar advantages of easy color rendering as type (d), using only one LED, while it has the drawbacks of complex blending of different phosphors and lack of efficient red phosphor. 2.3 Color Evaluation Indices What are the colors suitable for human sight? First, we have to define various colors of light on the basis of physical quantities (Phillips, 2006). The color of a light source can be described by the Commission Internatio- nale de l’Eclairage (CIE) chromaticity coordinates, calculated as follows. Fig. 4A displays the three color-matching functions, x"ðlÞ, y"ðlÞ, and z"ðlÞ, which define the “1931 CIE Standard Observer.” The function, y"ðlÞ is defined to be identical to the photopic luminous efficiency function, Vl, which describes the brightness perceived by the human eye of a light of a particular wavelength under day-brightness conditions. The other two color-matching functions, x"ðlÞ and z"ðlÞ, do not have such simple physical interpretations FIG. 4 CIE 1931 (1978) Color-matching functions (A) and CIE 1931 (x,y) chromaticity diagram and color temperature locus (B). The triangular area in the chromaticity diagram covers the gamut of colors defined by standard green (light gray in the print version), red (dark gray in the print version), and blue (dark gray in the print version) colors.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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