How phosphors work
The name Phosphor arose in the 19th century as a generic term for materials that glowed in the dark. At that time elemental white Phosphorous was the principle substance to exhibit this effect although certain minerals also had “phosphorescence”. The analysis and artificial synthesis of these minerals gradually yielded, over many years, the electronic transitions that were responsible for the effect.
Each luminescent material 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 by luminescent materials and re-emitted at specific wavelengths depending on the phosphor. The host crystal usually absorbs the energy and transmits it to activator ions embedded within the lattice. The activator ion then excites electrons into a higher energy level from which they return back to their original state with an emission of light with a wavelength corresponding to the energy difference between the two levels.
Temperature and Purity
On an atomic level, all ions in solid materials are in a perpetual state of vibration due to the energy associated with their temperature. Ambient temperature is ~300 oK above absolute zero where all vibration ceases. Efficient phosphors hold their ions very tightly so that energy is not lost in what is known as “non-radiative transitions”. Luminescent materials also have to be very pure for the same reason; impurity ions absorb or divert energy and “kill” the luminescent properties of a 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. (the higher the energy, the shorter the wavelength and vice versa).
Other phosphors have virtual line emission and 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 broad band emission.
The decay time (afterglow or persistence) of a phosphor is defined as the time taken for the emission intensity to decrease to 10% of the initial intensity before the excitation source was cut off. 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 are often defined by the 50% time rather than the 10% time, due to the difficulty of measuring ultra fast decay times.