text: NV centers emit bright red light (E→A transitions), if excited off-resonantly by visible green light (A →E transitions). This can be done with convenient light sources such as argon or krypton lasers, frequency doubled Nd:YAG lasers, dye lasers, or He-Ne lasers. Excitation can also be achieved at energies below that of zero phonon emission.
  As the relaxation time  from the excited state is small (~10 ns), the emission happens almost instantly after the excitation. At room temperature the NV center's optical spectrum exhibits no sharp peaks due to thermal broadening. However, cooling the NV centers with liquid nitrogen or liquid helium dramatically narrows the lines down to a width of a few MHz. At low temperature it also becomes possible to specifically address the zero-phonon line (ZPL).
  An important property of the luminescence from individual NV centers is its high temporal stability. Whereas many single-molecular emitters bleach (i.e. change their charge state and become dark) after emission of 10–10 photons, bleaching is unlikely for NV centers at room temperature. Strong laser illumination, however, may also convert some NV into NV centers.
  Because of these properties, the ideal technique to address the NV centers is confocal microscopy, both at room temperature and at low temperature.