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Fluorescence

Fluorescence

Fluorescence is an optical phenomenon wherein a material emits light in response to some external stimulus. Normally, the fluorescent light that is emitted is of a specific color or group of colors that is released when the material is bombarded with light in some other part of the color spectrum.

Certain minerals have a characteristic fluorescence pattern when hit with white light or ultraviolet light. Fluorite and calcite are two examples of fluorescent minerals. There are also many organic dye molecules with useful fluorescent properties. These molecules absorb light energy from external sources, and this energy causes some excitation of the electron orbitals in a process called pi-bonding. When the excited pi-bonds relax back to a lower energy state, photons of a specific wavelength are emitted in the process, giving rise to the fluorescent light. These organic dyes can be characterized by the wavelengths of light that they absorb (excitation wavelengths), and the wavelengths of light that they emit (emission wavelengths). The excitation and emission wavelengths are properties of each dye that are highly specific and reliable. Organic dyes tend to degrade over time as they are bombarded with light in a process called photodegradation. During photodegradation, the excited pi-bonds can break, rather than relaxing into their lower energy state. Organic fluorescent dyes have been in use for many years.

More recently, it has been discovered that very small particles of certain semiconductor materials also fluoresce, and the color of the fluorescence is dependent only on the size of the particles. These materials are referred to as quantum dots. Quantum dots absorb energy from a range of wavelengths, but the energy is not taken into pi-bonds. Rather, the fluorescence results from quantum mechanical interactions within the material. Smaller particles emit light on the blue end of the spectrum, whereas larger particles emit light on the red end of the spectrum. Because light energy is not absorbed into fragile pi-bonds, the rate of photodegradation is much lower for quantum dots compared with organic dyes, and thus the fluorescent signals are brighter and more durable.

There are a number ways in which fluorescence plays into forensic investigations. Biological materials sometimes have a characteristic fluorescent property that facilitates quick identification under UV examination. Semen stains, for example, may be identified by their characteristic fluorescence under ultraviolet light examination. Fluorescence of other biological samples can be brought about by chemical treatment to make their detection easier. Fingerprints can be treated with fluorescent powders to permit identification and detection even of relatively faint (latent) or degraded prints. Likewise, application of highly fluorescent materials, such as spy dust, permits tagging and tracking of suspects or agents across fairly wide areas by following the path of dispersal of the fluorescent agent as they drag and redistribute an unseen powder with their shoes or clothing. Certain chemicals, such as explosives and nerve agents, can sometimes be traced in the environment from their characteristic fluorescent spectral patterns. Examination of microscopic fibers for fluorescence can produce evidence linking suspects to crime scenes or other physical locations.

Fluorescence is a tool that allows evidence that would normally be invisible to come to light. It is a source of evidence only found with careful examination by those who are aware of its latent powers.

see also Chemical and biological detection technologies; Confocal microscopy; Microscopes; Semen and sperm.

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fluorescence

fluorescence (flŏŏrĕs´əns), luminescence 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 phosphorescence, 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. 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 lighting.

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fluorescence

fluo·res·cence / floŏ(ə)ˈresəns; flôrˈesəns/ • n. the visible or invisible radiation emitted by certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light. ∎  the property of absorbing light of short wavelength and emitting light of longer wavelength.

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fluorescence

fluorescence Kind of luminescence, in which an atom or molecule emits radiation when electrons within it pass back from a higher to their former, lower energy state. The term is restricted to the phenomenon in cases where the interval between absorption and emission is very short (less than 10−3s). See alsoPHOSPHORESCENCE; X-RAY FLUORESCENCE; and X-RAY FLUORESCENCE SPECTROMETRY.

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fluorescence

fluorescence Emission of radiation, usually light, from a substance when its atoms have acquired excess energy from a bombarding source of radiation, usually ultraviolet light or electrons. When the source of energy is removed, the fluorescence ceases. Mercury vapour is a fluorescent substance used in motorway lights; television tubes use fluorescent screens.

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fluorescence

fluorescence A kind of luminescence, in which an atom or molecule emits radiation when electrons within it pass back from a higher to their former, lower energy state. The term is restricted to the phenomenon in cases where the interval between absorption and emission is very short (less than 10−3 s).

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fluorescence

fluorescence A kind of luminescence, in which an atom or molecule emits radiation when electrons within it pass back from a higher to their former, lower-energy, state. The term is restricted to the phenomenon in cases where the interval between absorption and emission is very short (less than 10−3s).

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fluorescence

fluorescence (floo-er-ess-ĕns) n. the emission of light by a material as it absorbs radiation from outside. The radiation absorbed may be visible or invisible (e.g. ultraviolet rays or X-rays). See fluoroscope.
fluorescent adj.

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fluorescence

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