The shape of the zero-phonon line is Lorentzian with a width determined by the excited state lifetime T 10 according to the Heisenberg uncertainty principle. Without the influence of the lattice, the natural line width (full width at half maximum) of the chromophore is γ 0 = 1/ T 10 . The lattice reduces the lifetime of the excited state by introducing radiationless decay mechanisms. At absolute zero the lifetime of the excited state influenced by the lattice is T 1. Above absolute zero, thermal motions will introduce random perturbations to the chromophores local environment. These perturbations shift the energy of the electronic transition, introducing a temperature dependent broadening of the line width. The measured width of a single chromophore's zero phonon line, the homogeneous line width, is then γ h (T) ≥ 1/ T 1 .