Lighting, display, and imaging devices
Luminescence is defined broadly as the generation of light in excess of that radiated thermally. Man’s fascination with luminescence stems from when an otherwise invisible power is converted into visible light. The commercial importance of luminescence is ubiquitous, being manifest in lamps, displays, X-ray machines, etc.
What is luminescence science?
Materials that display luminescence are called phosphors. Commercial phosphors are generally inorganic compounds prepared as powders or thin films. The phosphor materials contain one or more impurity ions or activators, present in 1-100 mole percent concentrations. Typical activators are rare-earth or transition-metal ions undergoing s-p transitions, radicals such as tungstate or vandate groups, organic molecules, or electron-hole pairs.
Activator centers may be excited by high energy photons, such as X-rays or ultraviolet radiation from a mercury discharge (photoluminescence) or electron bombardment (cathodoluminescence). In addition, luminescence may be generated directly from an electric field (electroluminescence) or from infrared radiation (up-converting luminescence) or even mechanically (triboluminescence).
To have any technical importance, a luminescent material must be easily excited by the appropriate excitation and must have a high quantum efficiency — the ratio of the number of quanta absorbed to the number emitted. Non-radiative losses are commonly caused by interactions with the lattice vibrations and what are known as “killer” impurities. Further, the activator must convert the energy absorbed to a useful frequency of visible light. A suitable phosphor must maintain well under the excitation mode and must be easily manufacturable. The synthesis of efficient phosphors requires the best in high temperature chemistry. Device manufacturing involves still other sciences, such as thin film technology and suspension chemistry.
