The name Phosphor arose in the 19th century as a generic term for materials that glow in the dark. At the time, white Phosphorus was the principle substance to exhibit this effect, although certain minerals also had “phosphorescence”. Many years of analysis and artificial synthesis of these minerals followed. Leading to the understanding of the underlying electronic transitions that explain how phosphors work.
Each phosphor has its own individual characteristics, and so it is difficult to generalise the electronic processes that are taking place. However, a typical sequence of events is as follows:
Energy, in the form of light, electrons or particles, is absorbed and re-emitted at specific wavelengths. These wavelengths are dependent on the structure of the phosphor. The host crystal usually absorbs the energy and then transmits it to activator ions embedded within the crystal lattice. The energy passes into the electrons within the activator ion, causing them to move into a higher energy level. Upon relaxing back to a lower energy level, the electrons release energy as an emission of light. The wavelength corresponds to the energy difference between the two levels.
On an atomic level, all ions in solid materials are in a perpetual state of vibration due to the energy associated with their temperature. Vibration will only cease at absolute zero (approximately -273 oC). Efficient phosphors hold their ions very tightly, preventing the loss of energy through “non-radiative transitions”. They also have to be very pure for the same reason. Impurity ions absorb or divert energy and “kill” the luminescent properties of the material.
Some phosphors emit light with a “gaussian” distribution about a particular peak wavelength. This is due to the effects of temperature as described above. Vibration in the lattice causes some electrons to have more energy than others, so the light emitted from them will vary in wavelength about an average point.
Other phosphors have virtual line emission. This is because the electronic transitions occur within deeper orbitals that are immune to the temperature sensitive vibrations of the outer electron shells. Line emission is characteristic of rare-earth activators and the phosphors that result are very important materials in the phosphor industry. Rare earth ions may also give broadband emission.
The decay time (afterglow or persistence) of a phosphor is defined as the time taken, from when excitation ceases, for the emission intensity to decrease to 10% of its initial intensity. The decay time is dependent on the intrinsic properties of the material itself and/or the presence of “traps” or “killer” sites. Very fast decay phosphors use the 50% time rather than the 10% time, due to the difficulty of measuring ultra-fast decay times.