phosphordetection.co.uk is the specialist UV air disinfection arm of DaRo UV systems
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Wikipedia describes Fluorescence as the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, and the emitted light is in the visible region.
To summarise: Fluorescence is a physical property whereby light of one wavelength is converted to another usually longer wavelength. The conversion is virtually instantaneous and is difficult to to discern by eye especially if blue or ultraviolet excitation is required. The effect only persists after the excitation source is extinguished. The effect is called phosphorescence.
Fluorescence has many practical applications, including mineralogy, gemology, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors.