When SrAl2O4 365nm UV excitation light when , Eu2 + 4f → 5d transitions produce . Auxiliary active ions Dy3 + and adding , changing the shape of the lattice , the lattice distortion , resulting in the impurity level ( trap level ) , stones for driveway level is mainly produced by the D to join , and replaced by Sr2 + Dy3 + cause the generation of holes , the trap level is relatively uniform holes produced by the donor level . The energy level in the excited state of Eu2 + ground state level and between levels . When an electron is excited from the ground state to the excited state , glow gravels of the electronic transition back to the low level produces light, another part of the electrons through the relaxation process is stored in the trap energy level , when the electron trap levels absorbed energy, the re- stimulated Back excited states , transitions back to the ground state energy and light.