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general term applied to all forms of cool light, i.e., light emitted by sources other than a hot, incandescent body, such as a blackbody radiator. Luminescence is caused by the movement of electrons within a substance from more energetic states to less energetic
..... Click the link for more information. in which light of a visible color is emitted from a substance under stimulation or excitation by light or other forms of electromagnetic radiation or by certain other means. The light is given off only while the stimulation continues; in this the phenomenon differs from phosphorescencephosphorescence
, luminescence produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence is distinguished from fluorescence in that it continues even after the radiation causing it has ceased.
..... Click the link for more information. , in which light continues to be emitted after the excitation by other radiation has ceased. Fluorescence of certain rocks and other substances had been observed for hundreds of years before its nature was understood. Fluoresecence also occurs in some living organisms; some coral, reef fish, jellyfish, and other marine species as well as such terrestrial plants and animals as certain spiders and pitcher plants fluoresce. Probably the first to explain it was the British scientist Sir George G. Stokes, who named the phenomenon after fluorite, a strongly fluorescent mineral. Stokes is credited with the discovery (1852) that fluorescence can be induced in certain substances by stimulation with ultraviolet light. He formulated Stokes's law, which states that the wavelength of the fluorescent light is always greater than that of the exciting radiation, but exceptions to this law have been found. Later it was discovered that certain organic and inorganic substances can be made to fluoresce by activation not only with ultraviolet light but also with visible light, infrared radiation, X rays, radio waves, cathode rays, friction, heat, pressure, and some other excitants. Fluorescent substances, sometimes also known as phosphors, are used in paints and coatings, but their chief use is in fluorescent lightinglighting,
light produced by artificial means to allow visibility in enclosures and at night. For stage lighting, see scene design and stage lighting. Early Sources of Artificial Lighting
..... Click the link for more information. .
Fluorescence is generally defined as a luminescence emission that is caused by the flow of some form of energy into the emitting body, this emission ceasing abruptly when the exciting energy is shut off. In attempts to make this definition more meaningful it is often stated, somewhat arbitrarily, that the decay time, or afterglow, of the emission must be of the order of the natural lifetime for allowed radiative transitions in an atom or a molecule, which is about 10-8 s for transitions involving visible light. Perhaps a better distinction between fluorescence and its counterpart, phosphorescence, rests not on the magnitude of the decay time per se, but on the criterion that the fluorescence decay is temperature-independent.
In the literature of organic luminescence, the term fluorescence is used exclusively to denote a luminescence which occurs when a molecule makes an allowed optical transition. Luminescence with a longer exponential decay time, corresponding to an optically forbidden transition, is called phosphorescence, and it has a different special distribution from the fluorescence. See Phosphorescence
The decay time of fluorescent materials varies widely, from the order of 5 × 10-9 s for many organic crystalline materials up to 2 s for the europium-activated strontium silicate phosphor. Fluorescent materials with decay times between 10-9 and 10-7 s are used to detect and measure high-energy radiations, such as x-rays and gamma rays, and high-energy particles such as alpha particles, beta particles, and neutrons. These agents produce light flashes (scintillations) in certain crystalline solids, in solutions of many polynuclear aromatic hydrocarbons, or in plastics impregnated with these hydrocarbons. The so-called fluorescent lamps employ the luminescence of gases and solids in combination to produce visible light. See Luminescence
fluorescence(floo-ŏ-ress -ĕns) The transformation of photons of relatively high energy (i.e. high frequencies, especially ultraviolet frequencies) to lower-energy photons through interactions with atoms. It is also the lower-energy radiation that is produced by the process.
a luminescence that decays in a time τ ~ 10–8 –10–9 sec. The division of luminescence into fluorescence and phosphorescence is out of date, since it is an arbitrary distinction based on a qualitative measurement of the duration of luminescence. In terms of the mechanism for converting the excitation energy into visible emission, fluorescence, as a rule, is spontaneous luminescence; hence τ is determined by the lifetime of an atom in an excited state.
Resonance fluorescence, the frequency of which coincides with the frequency of the exciting radiation, is observed in atomic vapors (seeRESONANCE RADIATION). Molecules may fluoresce in highly rarefied vapors; an increase in the vapor pressure or the addition of foreign impurities may quench the fluorescence. Many organic substances, particularly aromatic compounds, fluoresce in liquid and solid solutions as well as in the crystalline state.
The spectra, polarization, and kinetics of fluorescence are associated with the structure and symmetry of molecules and with the nature of molecular interactions and depend, for example, on the concentration of solutions and the type of excitation. Fluorescence may be used, for example, to study crystal structure and excition processes in crystals (seeSPECTROSCOPY, CRYSTAL), the energy levels of molecules, the structure and interaction of molecules, and the processes by which excitation energy is transferred. Fluorescence is used in luminescence analysis, scintillation counters, and mineralogical research.
The decay time of fluorescence is measured by means of fluorometers.
REFERENCESSee references under .
of minerals, a luminescence that is excited in minerals by light, X rays, or electrons and that decays rapidly (within 10–2–10–1 sec) after the excitation ceases. The rapid decay of the luminescence distinguishes fluorescence from phosphorescence and thermoluminescence. The physical phenomenon of the fluorescence of minerals was first observed in fluorite, from which the term “fluorescence” is derived.
Fluorescence is characteristic of dielectric minerals and semiconductors that are transparent to visible light and to near-ultraviolet and infrared radiation. The fluorescence of minerals is associated with impurities or occasionally with intrinsic ions or complexes; such impurities, ions, or complexes form luminescence centers. Fluorescence is sometimes completely or partially quenched by certain isomorphic impurities, such as ions of bivalent iron.
The fluorescence of minerals is used in luminescence analysis to detect such minerals as scheelite, zircon, apatite, and uranites in mining excavations and to analyze microscopic impurities consisting of rare or dispersed elements, such as uranium or rare earths. It is also used for ore dressing by means of the identification of the useful component—for example, diamonds, fluorspar, or scheelite—on the basis of the component’s luminescence.
REFERENCEMarfunin, A. S. Spektroskopiia, liuminestsentsiia i radiatsionnye tsentry v mineralakh. Moscow, 1975.
B. S. GOROBETS