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1 IntroductionrnA wide range of luminescent materials have been studied by optical spectroscopy techniques. Prof. William M.Yen has made tremendous contribution to this research area. One of his interests in the recent years was in delocalization processes that strongly affect the luminescence efficiency[1]. Phosphors as luminescence center activated solids can be grouped into two categories: with and without delocalization. Phosphors in the first category have luminescence centers hosted by insulating crystals and do not have charge transport or transfer in their luminescent processes.