Fluorescence is an optical property unique to certain materials such as phosphors and other light-emitting compounds. Some materials tend to emit light upon absorbing some form of energy that could be optical, electrical, even mechanical or thermal. The term “fluorescence” in general is referred to the phenomenon involving light emission following the absorption of electromagnetic radiation, typically from ultraviolet or visible light. This can happen by an organic molecule or an inorganic compound with the subsequent emission of a photon. The term “scintillation” is also used sometimes to describe fluorescent behavior when a material is excited by ionizing radiation.The emitted light always has a lower energy (longer wavelength) than the original excitation energy. This is known as the Stokes shift.
Fluorescent materials can be inorganic such as phosphor crystals or organic pigments and dyes. They can be nano-crystals such as quantum dots (QDs) or large size crystals similar to gemstones. By incorporating or suspending such crystals into polymers or glass, it is possible to produce large fluorescent plates or luminescent 3D shapes. When the fluorescence remains persistent following removal of the excitation source, it is called phosphorescence. Phosphorescence is the main driving mechanism in glow-in-the-dark pigments.
Fluorescence can be in the form of visible light or it can be radiation invisible to the eyes such as ultraviolet or infrared luminescence. In the case where fluorescence is generated by absorbed photons of light, it can be produced using down-conversion or up-conversion processes. Down-conversion takes place when high energy photons are absorbed and converted to lower energy photons while up-conversion involves absorbing two lower energy photons in order to generate a single higher energy photon. Down-conversion is the process used to produce artificial lighting inside various lamps including fluorescent bulbs and and solid-state lighting (LED bulbs).
Fluorescence from large size single crystals
Amber and deep red fluorescence by different phosphors from down-conversion of blue LED light to longer color wavelengths