A good match between the chromophore and the host lattice leads to a uniform environment for all the chromophores and hence greatly reduces the inhomogeneous broadening of the electronic transition's pure electronic and vibronic lines. In addition to the weak inhomogeneous broadening of the transitions, the quasi-lines observed at very low temperatures are phonon -less transitions. Since phonons originate in the lattice, an additional requirement is weak chromophore-lattice coupling. Weak coupling increases the probability of phonon-less transitions and hence favors the narrow zero phonon lines. The weak coupling is usually expressed in terms of the Debye-Waller factor, where a maximum value of one indicates no coupling between the chromophore and the lattice phonons. The narrow lines characteristic of the Shpolskii systems are only observed at cryogenic temperatures because at higher temperatures many phonons are active in the lattice and all of the amplitude of the transition shifts to the broad phonon sideband. The original observation of the Shpolskii effect was made at liquid nitrogen temperature (77 kelvins), but using temperatures close to that of liquid helium (4.2 K) yields much sharper spectral lines and is the usual practice.