This broadening of the emission band with concentration is also explained by other radiation trapping effects [25] in which the emission of the excited Er3+ ion is re-absorbed by the non-excited Er3+ ion, considering the 4I13/2→4I15/2 transition and the following emission. This phenomenon always occurs in a typical 3-level system whenever the absorption and emission spectra overlap with each other. The re-absorption is also responsible for a small difference between experimental and calculated optical transitions [42,43], since the calculations do not consider this parameter while the direct measurement has it into account. Hence, the theoretical calculations are slightly higher than the observed in optical systems, more remarkably in highly concentrated systems.