Editing Fluorescence (section)

== Applications ==
=== Lighting ===
{{further|Fluorescent lamp|Blacklight}}
[[File:Art exhibition under black light.jpg|thumb|Fluorescent paint and plastic lit by UV-A lamps ([[blacklight]]). Paintings by Beo Beyond.]]

The common [[fluorescent lamp]] relies on fluorescence. Inside the [[glass]] tube is a partial vacuum and a small amount of [[mercury (element)|mercury]]. An electric discharge in the tube causes the mercury atoms to emit mostly ultraviolet light. The tube is lined with a coating of a fluorescent material, called the ''[[phosphor]]'', which absorbs ultraviolet light and re-emits visible light. Fluorescent [[lighting]] is more energy-efficient than [[incandescent]] lighting elements. However, the uneven [[spectrum]] of traditional fluorescent lamps may cause certain colors to appear different from when illuminated by incandescent light or [[daylight]]. The mercury vapor emission spectrum is dominated by a short-wave UV line at 254&nbsp;nm (which provides most of the energy to the phosphors), accompanied by visible light emission at 436&nbsp;nm (blue), 546&nbsp;nm (green) and 579&nbsp;nm (yellow-orange). These three lines can be observed superimposed on the white continuum using a hand spectroscope, for light emitted by the usual white fluorescent tubes. These same visible lines, accompanied by the emission lines of trivalent europium and trivalent terbium, and further accompanied by the emission continuum of divalent europium in the blue region, comprise the more discontinuous light emission of the modern trichromatic phosphor systems used in many [[compact fluorescent lamp]] and traditional lamps where better color rendition is a goal.<ref name="How Fluorescent Lamps Work">{{cite web|last=Harris|first=Tom|title=How Fluorescent Lamps Work|url=http://home.howstuffworks.com/fluorescent-lamp.htm|work=HowStuffWorks|publisher=Discovery Communications|access-date=27 June 2010|url-status=live|archive-url=https://web.archive.org/web/20100706040948/http://home.howstuffworks.com/fluorescent-lamp.htm|archive-date=6 July 2010|date=2001-12-07}}</ref>

Fluorescent lights were first available to the public at the [[1939 New York World's Fair]]. Improvements since then have largely been better phosphors, longer life, and more consistent internal discharge, and easier-to-use shapes (such as compact fluorescent lamps). Some [[High-intensity discharge lamp|high-intensity discharge (HID) lamps]] couple their even-greater electrical efficiency with phosphor enhancement for better color rendition.<ref>{{Cite book |last=Flesch |first=P. |url=https://www.worldcat.org/oclc/262693002 |title=Light and light sources: high-intensity discharge lamps |date=2006 |publisher=Springer-Verlag |isbn=978-3-540-32685-4 |location=Berlin |oclc=262693002}}</ref>

White [[light-emitting diode]]s (LEDs) became available in the mid-1990s as [[LED lamp]]s, in which blue light emitted from the [[semiconductor]] strikes phosphors deposited on the tiny chip. The combination of the blue light that continues through the phosphor and the green to red fluorescence from the phosphors produces a net emission of white light.<ref>{{Cite journal|last1=Chen|first1=Lei|last2=Lin|first2=Chun-Che|last3=Yeh|first3=Chiao-Wen|last4=Liu|first4=Ru-Shi|date=2010-03-22|title=Light Converting Inorganic Phosphors for White Light-Emitting Diodes|journal=Materials|volume=3|issue=3|pages=2172–2195|doi=10.3390/ma3032172|issn=1996-1944|pmc=5445896|bibcode=2010Mate....3.2172C|doi-access=free}}</ref>

[[Glow stick]]s sometimes utilize fluorescent materials to absorb light from the [[chemiluminescence|chemiluminescent]] reaction and emit light of a different color.<ref name="How Fluorescent Lamps Work"/>

=== Analytical chemistry ===
Many analytical procedures involve the use of a [[fluorometer]], usually with a single exciting wavelength and single detection wavelength. Because of the sensitivity that the method affords, fluorescent molecule concentrations as low as 1 part per trillion can be measured.<ref>{{Cite journal | doi = 10.1006/abio.1993.1020| title = Fluorometric Assay Using Dimeric Dyes for Double- and Single-Stranded DNA and RNA with Picogram Sensitivity| journal = Analytical Biochemistry| volume = 208| issue = 1| pages = 144–150| year = 1993| last1 = Rye | first1 = H. S. | last2 = Dabora | first2 = J. M. | last3 = Quesada | first3 = M. A. | last4 = Mathies | first4 = R. A. | last5 = Glazer | first5 = A. N. | pmid=7679561}}</ref>

Fluorescence in several wavelengths can be detected by an [[Chromatography detector|array detector]], to detect compounds from [[High-performance liquid chromatography|HPLC]] flow. Also, [[Thin layer chromatography|TLC]] plates can be visualized if the compounds or a coloring reagent is fluorescent. Fluorescence is most effective when there is a larger ratio of atoms at lower energy levels in a [[Boltzmann distribution]]. There is, then, a higher probability of excitement and release of photons by lower-energy atoms, making analysis more efficient.

=== Spectroscopy ===
{{Main|Fluorescence spectroscopy}}
Usually the setup of a fluorescence assay involves a light source, which may emit many different wavelengths of light. In general, a single wavelength is required for proper analysis, so, in order to selectively filter the light, it is passed through an excitation monochromator, and then that chosen wavelength is passed through the sample cell. After absorption and re-emission of the energy, many wavelengths may emerge due to [[Stokes shift]] and various [[electron transition]]s. To separate and analyze them, the fluorescent radiation is passed through an emission [[monochromator]], and observed selectively by a detector.<ref>{{cite book|author=Harris, Daniel C.|title=Exploring chemical analysis|url=https://books.google.com/books?id=x5eEW76lizEC|date=2004|publisher=Macmillan|isbn=978-0-7167-0571-0|url-status=live|archive-url=https://web.archive.org/web/20160731220213/https://books.google.com/books?id=x5eEW76lizEC|archive-date=31 July 2016}}</ref>

=== Lasers ===
[[File:Dye laser alignment intra-cavity beam @ 589nm.jpg|thumb|The internal cavity of a dye laser tuned to 589&nbsp;nm. The green beam from a frequency-doubled [[Nd:YAG laser]] causes the dye solution to fluoresce in yellow, creating a beam between the array of mirrors.]]
[[Laser]]s most often use the fluorescence of certain materials as their active media, such as the red glow produced by a [[ruby laser|ruby]] (chromium sapphire), the infrared of [[titanium-sapphire laser|titanium sapphire]], or the unlimited range of colors produced by [[dye laser|organic dyes]]. These materials normally fluoresce through a process called [[spontaneous emission]], in which the light is emitted in all directions and often at many discrete spectral lines all at once. In many lasers, the fluorescent medium is [[laser pumping|"pumped"]] by exposing it to an intense light source, creating a [[population inversion]], meaning that more of its atoms become in an excited state (high energy) rather than at ground state (low energy). When this occurs, the spontaneous fluorescence can then induce the other atoms to emit their photons in the same direction and at the same wavelength, creating [[stimulated emission]]. When a portion of the spontaneous fluorescence is trapped between two mirrors, nearly all of the medium's fluorescence can be stimulated to emit along the same line, producing a laser beam.<ref>''Fundamental and Details of Laser Welding'' by Seiji Katayama – Springer 2020 p. 3–5</ref>

=== Biochemistry and medicine ===
{{Main|Fluorescence in the life sciences}}
[[Image:FluorescentCells.jpg|thumb|right|[[Endothelium|Endothelial cells]] under the microscope with three separate channels marking specific cellular components]]
Fluorescence in the life sciences is used generally as a non-destructive way of tracking or analysis of biological molecules by means of the fluorescent emission at a specific frequency where there is no background from the excitation light, as relatively few cellular components are naturally fluorescent (called intrinsic or [[autofluorescence]]).
In fact, a [[protein]] or other component can be "labelled" with an extrinsic [[fluorophore]], a fluorescent [[dye]] that can be a small molecule, protein, or quantum dot, finding a large use in many biological applications.<ref name=Lakowicz-1999/>{{rp|style=ama|p= {{mvar|xxvi}} }}

The quantification of a dye is done with a [[spectrofluorometer]] and finds additional applications in:

==== Microscopy ====
* When scanning the fluorescence intensity across a plane one has [[fluorescence microscope|fluorescence microscopy]] of tissues, cells, or subcellular structures, which is accomplished by labeling an antibody with a fluorophore and allowing the antibody to find its target antigen within the sample. Labelling multiple antibodies with different fluorophores allows visualization of multiple targets within a single image (multiple channels). DNA microarrays are a variant of this.
* Immunology: An antibody is first prepared by having a fluorescent chemical group attached, and the sites (e.g., on a microscopic specimen) where the antibody has bound can be seen, and even quantified, by the fluorescence.
* FLIM ([[Fluorescence Lifetime Imaging Microscopy]]) can be used to detect certain bio-molecular interactions that manifest themselves by influencing fluorescence lifetimes.
* Cell and molecular biology: detection of [[colocalization]] using fluorescence-labelled antibodies for selective detection of the antigens of interest using specialized software such as ImageJ.

==== Other techniques ====
* FRET ([[Förster resonance energy transfer]], also known as [[fluorescence resonance energy transfer]]) is used to study protein interactions, detect specific nucleic acid sequences and used as biosensors, while fluorescence lifetime (FLIM) can give an additional layer of information.
* Biotechnology: [[biosensors]] using fluorescence are being studied as possible [[Fluorescent glucose biosensors]].
* Automated sequencing of [[DNA]] by the [[chain termination method]]; each of four different chain terminating bases has its own specific fluorescent tag. As the labelled DNA molecules are separated, the fluorescent label is excited by a UV source, and the identity of the base terminating the molecule is identified by the wavelength of the emitted light.
* FACS ([[fluorescence-activated cell sorting]]). One of several important [[cell sorting]] techniques used in the separation of different cell lines (especially those isolated from animal tissues).
* DNA detection: the compound [[ethidium bromide]], in aqueous solution, has very little fluorescence, as it is quenched by water. Ethidium bromide's fluorescence is greatly enhanced after it binds to DNA, so this compound is very useful in visualising the location of DNA fragments in [[agarose gel electrophoresis]]. Intercalated ethidium is in a hydrophobic environment when it is between the base pairs of the DNA, protected from quenching by water which is excluded from the local environment of the intercalated ethidium. Ethidium bromide may be carcinogenic – an arguably safer alternative is the dye [[SYBR Green]].
* FIGS ([[Fluorescence image-guided surgery]]) is a medical imaging technique that uses fluorescence to detect properly labeled structures during surgery.
* [[Intravascular fluorescence]] is a catheter-based medical imaging technique that uses fluorescence to detect high-risk features of atherosclerosis and unhealed vascular stent devices.<ref name="pmid20210433">{{cite journal|author-link3=Vasi;is Ntziachristos|vauthors=Calfon MA, Vinegoni C, Ntziachristos V, Jaffer FA | title=Intravascular near-infrared fluorescence molecular imaging of atherosclerosis: toward coronary arterial visualization of biologically high-risk plaques. | journal=J Biomed Opt | year= 2010 | volume= 15 | issue= 1 | pages= 011107–011107–6 | pmid=20210433 | doi=10.1117/1.3280282 | pmc=3188610 | bibcode=2010JBO....15a1107C }}</ref> Plaque autofluorescence has been used in a first-in-man study in coronary arteries in combination with [[optical coherence tomography]].<ref name="pmid26971006">{{cite journal|vauthors=Ughi GJ, Wang H, Gerbaud E, Gardecki JA, Fard AM, Hamidi E, etal |title=Clinical Characterization of Coronary Atherosclerosis With Dual-Modality OCT and Near-Infrared Autofluorescence Imaging | journal=JACC Cardiovasc Imaging | year= 2016 | volume=  9| issue=  11| pages=  1304–1314| pmid=26971006 | pmc=5010789 | doi=10.1016/j.jcmg.2015.11.020}}</ref> Molecular agents has been also used to detect specific features, such as stent [[fibrin]] accumulation and enzymatic activity related to artery inflammation.<ref name="pmid26685129">{{cite journal|vauthors=Hara T, Ughi GJ, McCarthy JR, Erdem SS, Mauskapf A, Lyon SC, etal | title=Intravascular fibrin molecular imaging improves the detection of unhealed stents assessed by optical coherence tomography in vivo | journal=Eur Heart J | year= 2015 | volume=  38| issue=  6| pages=  447–455| pmid=26685129 | pmc=5837565 | doi=10.1093/eurheartj/ehv677}}</ref>
* SAFI (species altered fluorescence imaging) an imaging technique in [[electrokinetic phenomena|electrokinetics]] and [[microfluidics]].<ref>
{{cite journal
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 |url         = https://microfluidics.stanford.edu/Publications/ParticleTracking_Diagnostics/Shkolnikov_A%20method%20for%20non-invasive%20full-field%20imaging%20and%20quantification%20of%20chemical%20species.pdf
 |year        = 2013
 |last1     = Shkolnikov
 |first1      = V
 |title       = A method for non-invasive full-field imaging and quantification of chemical species
 |journal     = Lab on a Chip
 |volume      = 13
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 |archive-date = 5 March 2016
}}</ref> It uses non-electromigrating dyes whose fluorescence is easily quenched by migrating chemical species of interest. The dye(s) are usually seeded everywhere in the flow and differential quenching of their fluorescence by analytes is directly observed.
* Fluorescence-based assays for screening [[Toxicity|toxic]] chemicals.  The optical assays consist of a mixture of environment-sensitive fluorescent dyes and human skin cells that generate fluorescence spectra patterns.<ref name="Moczko2016">
{{cite journal
 | pmid = 27653274
 | pmc = 5031998
 | year = 2016
 | last1 = Moczko | first1 = E
 | title = Fluorescence-based assay as a new screening tool for toxic chemicals
 | journal = Scientific Reports
 | volume = 6
 | page = 33922
 | last2 = Mirkes | first2 = EM
 | last3 = Cáceres | first3 = C
 | last4 = Gorban | first4 = AN
 | last5 = Piletsky | first5 = S
 | doi = 10.1038/srep33922
 | bibcode = 2016NatSR...633922M
}}</ref> This approach can reduce the need for [[Animal testing|laboratory animals]] in biomedical research and pharmaceutical industry.
* Bone-margin detection: [[Alizarin|Alizarin-stained]] specimens and certain fossils can be lit by fluorescent lights to view anatomical structures, including bone margins.<ref>{{Cite journal|last1=Smith|first1=W. Leo|last2=Buck|first2=Chesney A.|last3=Ornay|first3=Gregory S.|last4=Davis|first4=Matthew P.|last5=Martin|first5=Rene P.|last6=Gibson|first6=Sarah Z.|last7=Girard|first7=Matthew G.|date=2018-08-20|title=Improving Vertebrate Skeleton Images: Fluorescence and the Non-Permanent Mounting of Cleared-and-Stained Specimens|journal=Copeia|language=en-US|volume=106|issue=3|pages=427–435|doi=10.1643/cg-18-047|issn=0045-8511|doi-access=free}}</ref>

=== Forensics ===
[[Fingerprint]]s can be visualized with fluorescent compounds such as [[ninhydrin]] or DFO ([[1,8-Diazafluoren-9-one]]). Blood and other substances are sometimes detected by fluorescent reagents, like [[fluorescein]]. [[Fiber]]s, and other materials that may be encountered in [[Forensic science|forensics]] or with a relationship to various [[collectible]]s, are sometimes fluorescent.

=== Non-destructive testing ===
[[Fluorescent penetrant inspection]] is used to find cracks and other defects on the surface of a part. [[Dye tracing]], using fluorescent dyes, is used to find leaks in liquid and gas plumbing systems.

=== Signage ===
[[File:New fluorescent school zone sign.JPG|thumb|A road sign, with the words "school zone" on a fluorescent-yellow background]]
Fluorescent colors are frequently used in [[signage]], particularly road signs. Fluorescent colors are generally recognizable at longer ranges than their non-fluorescent counterparts, with fluorescent orange being particularly noticeable.<ref>Hawkins, H. Gene; Carlson, Paul John and Elmquist, Michael (2000) [http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/2962-S.pdf "Evaluation of fluorescent orange signs"] {{webarchive|url=https://web.archive.org/web/20160304032241/http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/2962-S.pdf |date=4 March 2016 }}, Texas Transportation Institute Report 2962-S.</ref> This property has led to its frequent use in safety signs and labels.

=== Optical brighteners ===
{{Main|Optical brightener}}
Fluorescent compounds are often used to enhance the appearance of fabric and paper, causing a "whitening" effect. A white surface treated with an optical brightener can emit more visible light than that which shines on it, making it appear brighter. The blue light emitted by the brightener compensates for the diminishing blue of the treated material and changes the hue away from yellow or brown and toward white. Optical brighteners are used in laundry detergents, high brightness paper, cosmetics, [[high-visibility clothing]] and more.