Fluorescence is the emission of light by a material, after it has absorbed any form of electro-magnetic energy, usually in the form of light. It is caused by the process, in which an electron, in an atom or a molecule, after absorbing the energy and becoming excited (moving to a higher energy level), loses the excess energy by emitting a photon. Since usually the light emitted by the material is of a longer wavelength (lower frequency, lower energy) than the light initially absorbed by the material (this being because some of the energy is lost in non-radiating processes), it is possible, for example, to expose some materials to UV radiation, and achieve the emission of visible light from those materials.
The fluorescent qualities of a certain material are usually defined by the ratio of photons absorbed to photons emitted. The highest possible ratio is 1, when for every photon absorbed one is emitted, but even materials which possess a ratio of 0.1 are considered quite fluorescent.
This phenomenon acquired quite a number of technological applications, first and foremost of which is the fluorescent and LED lighting, producing white, energy-efficient light.
In science, fluorescence is used in chromatography and spectroscopy – both techniques that are used to chemical composition of materials. Chromatography uses fluorescent chemical indicators to discern composition of a mixture – the indicators (or analytes) which interacted most with the mixture will emit more light, thus indicating the mixture's components and their ratio in it. Spectroscopy is mainly used to recreate structures of molecules, by superimposing the molecule's fluorescent response to being lighted with certain wavelengths. This gives researchers the knowledge as to the atoms composing the molecule and the structure of the bonds between them. In recent years, spectroscopy can also be used in diagnosis of skin cancer.
Fluorescence is also widely used in life science, since it is a non-destructive way of analyzing biological molecules. For example, by tagging different nutrient molecules with different fluorescent "tags", it is possible to see where each material is used in constructing the cell. It is also used in tracking interactions between proteins, and interactions between antigens and antibodies.
It should be obvious by now that the most important trait of fluorescent materials is them being luminous, and thus easily discernable from their environment. Thus, the quality and accuracy of any test or experiment that uses fluorescence as a technique depends greatly on using the right substrate – namely, one of low fluorescence or, preferably, no fluorescence at all.
There is a great challenge in producing a light absorbing background, upon which fluorescent materials will be seen as clearly as possible, while keeping the absorbed light from being emitted back. The process of lowering a material's fluorescence is called fluorescent quenching, and it is the corner stone in producing any and all scientific and medical equipment that uses fluorescent techniques.