The photoluminescence band of the NV -center in diamond shows a strong optical transition with a sharp zero phonon line (ZPL) at 1.945 eV (637 nm) along with a phonon side band in the absorption spectra with maximum intensity peak at ∼2.18 eV. Davies and Hamer analyzed this experimental ZPL and found that the defect center has a trigonal C 3V symmetry and the ZPL is associated with a strong optical transition between the triplet ground state 3 A 2 and the first excited triplet state 3 E. The defect center also exhibits a weak band in the infrared region with a ZPL at 1.19 eV (1046 nm) along with a phonon side band with maximum intensity peak at 1.26 eV 46 in the absorption spectra. This ZPL corresponds to a transition between two intermediate metastable singlet states -1 A 1 and 1 E located between the two triplets. The optical transition involving the m s = 0 sublevels of the ground and excited triplet states is spin conserving and therefore the final state after deexcitation is the m s = 0 sublevel of the ground state. However, the excitation involving the m s = ±1 levels follows an intersystem crossing path via the intermediate singlet states and finally decays to the m s = 0 sublevel of the ground state. The intermediate singlets thus play a crucial role in the optical initialization of defect spin state through the spin polarization process.