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Rare Earth-Doped Phosphors for White LEDs Chapter 278 9 (Schanda, 2007). Thus, Ri gives the CRI with respect to each sample. Then, Ra is obtained by calculating the arithmetic mean of Ri. However, it was realized that the definition of CRI or Ra is far from perfect, and several improve- ments were proposed, details of which are not described here (Davis and Ohno, 2005). 3 PHOSPHOR REQUIREMENTS FOR LED LIGHTING The phenomena which involve absorption of energy and subsequent emission of light are classified generically under the term luminescence. Phosphors are luminescent materials that emit light when excited by radiation. In lumines- cent materials, absorption of energy takes place either by the host lattice or by intentionally doped impurities (localized emission center) such as rare earth and transition metal ions. The 4f electronic energy levels of rare earth ions are the mostly responsible for luminescence phenomena via both transi- tions of f–f and f–d/d–f. The energy levels are not affected much by the envi- ronment because the 4f electrons are shielded from external electric fields by the outer 5s2 and 5p6 electrons. This feature is in contrast with transition metal ions, whose 3d electrons, located in an outer orbit, are considerably affected by the environment or crystal electric field. Among the requirements for the development of LED lighting, as pointed out before, issues regarding phosphor materials are the following: (a) high quantum efficiencies of emission; (b) excitation spectrum matched to LED emission wavelengths; (c) short photoluminescence lifetimes; and (d) absence of thermal quenching at operating temperatures. Here, restricting our attention to rare earth ions, the items that are important for understanding the luminescence phenomena are introduced. 3.1 Features of Rare Earth Ions with Respect to Luminescence 3.1.1 Configurational Coordinate Diagram First, let us explain the configurational coordinate diagram because it is very useful for describing the luminescence mechanism of rare earth ions. In solids, the emission of rare earth ions is observed at different spectral position from the absorption. The difference between the absorption and emission wavelength is described as the Stokes shift. The configurational coordinate diagram can well explain not only Stokes shift but also optical properties of a localized center on the basis of potential curves (Yamamoto, 2006), as shown in Fig. 5. The total energy U of the activator is plotted as a function of configurational coordinates. Here, the total energy U means the sum of the electron energy and ion energy for an isolated molecule, which is regarded as the environment around an emission center ion. The configurational coor- dinate can describe one aspect of the geometrical configuration of the activePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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