sentences:  Many phosphors tend to lose efficiency gradually by several mechanisms.The activators can undergo change of valence (usually oxidation), the crystal lattice degrades, atoms – often the activators – diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing either the exciting or the radiated energy, etc.The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature; moisture impairs phosphor lifetime very noticeably as well.