3.2 Luminescent Pigments

As stated above, there is no intention to provide any detailed discussions of the theory of luminescent phenomena; however, a basic understanding is essential. A simplified outline, the vibrational levels at each state being omitted, of the energy diagram for fluorescence and phosphorescence is shown in Figure 3.1. On absorption of light the molecule is excited from its ground level state S ₀ to the first excited singlet state S ₁. Energy can be dissipated from this state by collision deactivation causing a reduction in the intensity of the resultant emitted light. If the molecule returns to the ground state rapidly then fluorescence occurs at a longer wavelength than absorption due to a loss of vibrational energy in the excited state; this difference in absorption is called the Stokes shift. Another process involves intersystem crossing from S ₁ to the triplet state. Energy emission from this triplet state T ₁ back to the ground state S ₀ causes phosphorescence. Phosphorescence always occurs at a longer wavelength than fluorescence because the energy difference between T ₁ and S ₀ is always lower than that between S ₁ and S ₀.

Figure 3.1: Schematic energy diagram for phosphorescence and fluorescence.

The vast majority of commercially important luminescent phosphors or pigments are inorganic. The use of organic phosphors is usually restricted to those applications where particle size requirements limit the use of inorganic materials. The luminescent materials are often called phosphorescent...