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[[Image:Trees and sunshine.JPG|thumb|right|333px|Light (a form of radiant energy) observed in a forest]]
'''Radiant energy''' is the [[energy]] of [[electromagnetic wave]]s, or sometimes of other forms of [[radiation]].<ref>"''[http://www.its.bldrdoc.gov/fs-1037/dir-029/_4341.htm Radiant energy]''". [[Federal standard 1037C]]</ref> The quantity of radiant energy may be calculated by [[Integral|integrating]] [[radiant flux]] (or [[Power (physics)|power]]) with respect to [[time]] and, like all forms of energy, its [[SI]] unit is the [[joule]].  The term is used particularly when radiation is emitted by a source into the surrounding environment. 

==Terminology use and history==
The term "radiant energy" is most commonly used in the fields of [[radiometry]], [[solar energy]], [[heating]] and [[lighting]], but is also sometimes used in other fields (such as [[telecommunications]]). Radiant energy may or may not affect the eye and produce vision.<ref>George Frederick Barker, ''Physics: Advanced Course'', page 367</ref> In modern applications involving transmission of power from one location to another, "radiant energy" is sometimes used to refer to the electromagnetic waves ''themselves'', rather than their ''energy'' (a property of the waves).  In the past, the term "electro-radiant energy" has also been used.<ref>Examples: {{patent|US|1005338|"Transmitting apparatus"}}, {{patent|US|1018555|"Signaling by electroradiant energy"}}, and {{patent|US|1597901|"Radio apparatus"}}.</ref>

Historically, the propagation of electromagnetic radiation was presumed to rely on a medium filling all space, known as the [[aether theories|aether]].<ref>Thomas Preston, "''[http://books.google.com/books?vid=0Ip-GtK-wSYf2ItVGW&id=QgseAAAAMAAJ The Theory of Light]''". Macmillan, 1901. Page 542.</ref><ref>George Frederick Barker, ''Physics: Advanced Course'', page 365.</ref><ref>Frederick Booth, ''Radiant Energy and the Ophthalmic Lens''.</ref> Electromagnetic waves were presumed to propagate through this medium by inducing transverse electric and magnetic stresses and strains, analogous to those induced by [[S-wave|shear wave]]s propagating through a physical medium.<ref>{{cite book| first=Louis| last=Bell| title=Electric Power Transmission; a Practical Treatise for Practical Men| publisher=Electrical World and Engineer| year=1901 |pages=p. 10 |url=http://books.google.com/books?id=hSYKAAAAIAAJ&pg=RA3-PA110&lpg=RA3-PA110&dq=%22electric+power+transmission+a+practical+treatise+for+practical+men%22&source=web&ots=FTKTW8smJm&sig=8kcwxaAWKmm5-1ysBFR52oQiRik#PRA1-PA10,M1 |accessdate=2007-02-15}}</ref> In modern times, the propagation of electromagnetic waves has been shown not to require any physical medium, although some interpretations of [[general relativity]] can be viewed as implying that space acts as a kind of non-physical "medium" for light.<ref>[[Albert Einstein]] said that space is "endowed with physical quantities," but that "this ether may not be thought of as endowed with the quality characteristic of ponderable media [...] The idea of motion may not be applied to it." (from "[http://www.mountainman.com.au/aether_0.html  Ether and the Theory of Relativity]", an address delivered [[1920-05-05]] at the University of Leyden).</ref><ref>[[Paul Dirac|P.A.M. Dirac]], "Is there an ether?" ''Nature'', '''168''', 906 (1951). Dirac wrote, "We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an ether."</ref>

==Analysis==
[[Image:TrigaReactorCore.jpeg|thumb|333px|[[Cherenkov radiation]] glowing in the core of a [[TRIGA|TRIGA reactor]].]]
Because electromagnetic (EM) radiation can be conceptualized as a stream of [[photon]]s, radiant energy can be viewed as the energy carried by these photons. Alternatively, EM radiation can be viewed as an electromagnetic wave, which carries energy in its oscillating electric and magnetic fields. These two views are completely equivalent and are reconciled to one another in [[quantum field theory]] (see [[wave-particle duality]]). 

EM radiation can have various [[frequency|frequencies]]. The bands of frequency present in a given EM signal may be sharply defined, as is seen in [[Emission spectrum|atomic spectra]], or may be broad, as in [[blackbody radiation]]. In the photon picture, the energy carried by each photon is proportional to its frequency. In the wave picture, the energy of a monochromatic wave is proportional to its [[intensity]]. This implies that if two EM waves have the same intensity, but different frequencies, the one with the higher frequency "contains" fewer photons, since each photon is more energetic.

When EM waves are [[Absorption (optics)|absorbed]] by an object, the energy of the waves is typically converted to heat. This is a very familiar effect, since sunlight warms surfaces that it irradiates. Often this phenomenon is associated particularly with [[infrared]] radiation, but any kind of electromagnetic radiation will warm an object that absorbs it. EM waves can also be [[Reflection (physics)|reflected]] or [[scattering|scattered]], in which case their energy is redirected or redistributed as well.

==Open systems==
Radiant energy is one of the mechanisms by which energy can enter or leave an [[Open system (systems theory)|open system]].<ref>Moran, M.J. and Shapiro, H.N., ''Fundamentals of Engineering
Thermodynamics'', Chapter 4. "Mass Conservation for an Open System", 5th Edition, John Wiley and Sons. ISBN 0471274712.</ref><ref>Robert W. Christopherson, ''Elemental Geosystems'', Fourth Edition. Prentice Hall, 2003. Pages 608. ISBN 0131015532</ref><ref>James Grier Miller and Jessie L. Miller, ''[http://www.newciv.org/ISSS_Primer/asem22jm.html The Earth as a System]''.</ref> Such a system can be man-made, such as a [[solar energy]] collector, or natural, such as the [[Earth's atmosphere]]. In [[geophysics]], transparent [[greenhouse gas]]es trap the sun's radiant energy (at certain wavelengths), allowing it to penetrate deep into the atmosphere or all the way to the Earth's surface, where they are re-emitted as longer wavelength radiation (chiefly infrared radiation). Radiant energy is produced in the sun as a result of [[nuclear fusion]].<ref>''[http://assets.cambridge.org/97805217/91656/excerpt/9780521791656_excerpt.pdf Energy transformation]''. assets.cambridge.org. (excerpt)</ref>

==Applications==
Radiant energy, as well as [[convective|convective energy]] and [[conductive|conductive energy]], is used for [[radiant heating]].<ref>{{patent|US|1317883|"Method of generating radiant energy and projecting same through free air for producing heat"}}</ref> It can be generated electrically by [[infrared]] lamps, or can be absorbed from [[sunlight]] and used to heat water. The heat energy is emitted from a warm element (floor, wall, overhead panel) and warms people and other objects in rooms rather than directly heating the air. The internal air temperature for radiant heated buildings may be lower than for a conventionally heated building to achieve the same level of body comfort (the perceived temperature is actually the same).

[[Image:PhotoelectricEffect(Tesla).png|thumb|333px|right|Photoelectric motor, US685957<br>
Radiant energy falling on a insulated conductor connected to a capacitor: the capacitor charges electrically.]]
Various other applications of radiant energy have been devised.<ref>Class 250, Radiant Energy, USPTO. March 2006.</ref> These include: 
* Treatment and inspection
* Separating and sorting
* Medium of control
* Medium of communication
Many of these applications involve a source of radiant energy and a detector that responds to that radiation and provides a signal representing some characteristic of the radiation. Radiant energy detectors produce responses to incident radiant energy either as an increase or decrease in [[electric potential]] or [[electric current|current]] flow or some other perceivable change, such as exposure of [[photographic film]]. 

One of the earliest wireless [[telephone]]s to be based on radiant energy was invented by [[Nikola Tesla]]. The device used transmitters and receivers whose resonances were tuned to the same frequency, allowing communication between them. In 1916, he recounted an experiment he had done in 1896.<ref name="Anderson">Anderson, Leland I. (editor), ''Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony and Transmission of Power'', 2002, ISBN 1-893817-01-6.</ref> He recalled  that "Whenever I received the effects of a transmitter, one of the simplest ways [to detect the wireless transmissions] was to apply a magnetic field to currents generated in a conductor, and when I did so, the low frequency gave audible notes."

==SI radiometry units==
{{SI radiometry units}}

==See also==
{{portal|Energy}}
;Main concepts: [[Luminous energy]], [[Power (physics)|Power]], [[Radiometry]], [[Federal Standard 1037C]], [[Transmission (telecommunications)|Transmission]], [[Electrostatics]], [[Ionizing radiation]], [[Non-ionizing radiation]]
;Science:[[Photoelectric effect]], [[Open system (systems theory)|Open system]], [[Cosmic microwave background radiation]]
;Photonic devices:[[Photodetector]], [[Photocell]], [[Photoelectric cell]]

{{radio_spectrum}}
{{EMSpectrum}}

==Notes==
<div class="references-small">
<references/>
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==References and further reading==
===General Information===
<div class="references-small">
* Patents > Guidance, Tools, and Manuals >> Classification >>> Class Definition : [http://www.uspto.gov/go/classification/uspc250/defs250.htm Class 250, Radiant Energy], USPTO. March 2006.
* Finke, R. C., "''Direct conversion of infrared radiant energy for space power applications''". In R and D Associates Proc. of the AFOSR Spec. Conf. on Prime-Power for High Energy Space Systems, Vol. 2 22 p (SEE N83-15860 06-44). 1982.
* Lang, K. R. (1999). [http://books.google.com/books?id=HlGIXqzVEAgC Astrophysical formulae]. Astronomy and astrophysics library. Berlin: Springer.
* Whittaker, E. T. (1910). ''A history of the theories of aether and electricity from the age of Descartes to the close of the nineteenth century''. Dublin University Press series. London: Longmans, Green and Company.
* Caverly, Donald Philip, "''Primer of electronics and radiant energy''"  New York, McGraw-Hill, 1952. 
* Hardis, Jonathan E., "''[http://nvl.nist.gov/pub/nistpubs/sp958-lide/025-027.pdf Visibility of Radiant Energy]''". [[Portable Document Format|PDF]].
* [http://www.schorsch.com/kbase/glossary/radiant_energy.html Lighting Design Knowledgebase]
* Sir Joseph Larmor, ''[http://books.google.com/books?vid=0K-ZW40fafKNaKAkdE&id=15AIAAAAMAAJ Aether and Matter: A Development of the Dynamical Relations of the Aether to Material Systems on the Basis of the Atomic Constitution of Matter]''. University press, 1900. 365 pages.
* ''Philosophical magazine''. (1798). London: Taylor & Francis. 
* Hegeler, E. C., & Carus, P. (1890). ''The Monist''. La Salle, Ill. [etc.]: Published by Open Court for the Hegeler Institute. 
* George Frederick Barker, ''[http://books.google.com/books?vid=0hXR-OKgxbYIeyr8l5-&id=bKEAAAAAMAAJ Physics: Advanced Course]''. Henry Holt and Company, 1893. 902 pages. 
* Frederick Booth, ''[http://books.google.com/books?vid=0Vl0QNaG_TLIdhglN5yZ&id=UFI0Zy3D0GsC Radiant Energy and the Ophthalmic Lens]''. P. Blakiston's Son & Co., 1921. 226 pages.
* Thomas O'Conor Sloane, ''[http://books.google.com/books?vid=0S94VxA1OrVBaWjM7o7&id=sD4JAAAAIAAJ Electricity Simplified: The Practice and Theory of Electricity]''. The N.W. Henley publishing co., 1905. 172 pages.
* "[http://books.google.com/books?vid=0CZkDzy7hqYWpJVqcWCZAHZ&id=5aUOAAAAIAAJ&pg=PT17 Hermann von Helmholtz]" (Obiturary). Royal Society (Great Britain). (1854). Proceedings of the Royal Society of London. London: Printed by Taylor and Francis.
* Whittaker, E. T., ''[http://links.jstor.org/sici?sici=0025-5572(192904)2%3A14%3A200%3C401%3AWIE%3E2.0.CO%3B2-K What Is Energy]?''. The Mathematical Gazette, 1929.
* Peter Michael Harman, George Basalla, and Owen Hannaway, "''[http://books.google.com/books?id=NtHlec13w9YC Energy, Force and Matter: The Conceptual Development of Nineteenth-century Physics]''". Cambridge University Press, 1982.
* U. Fano, ''[http://prola.aps.org/abstract/PR/v103/i5/p1202_1 Atomic Theory of Electromagnetic Interactions in Dense Materials]''. 8 May 1956.
* JM Dawson, ''Particle simulation of plasmas''. Reviews of Modern Physics, 1983.
* Glenn R. Elion, ''[http://books.google.com/books?id=c1jqBKKKDkwC Electro-Optics Handbook]''. CRC Press Technology & Industrial Arts, 1979. ISBN 0824768795
* Draper, J. W. (1878). ''[http://books.google.com/books?vid=0mpXqc8oysvOALpqxEh&id=DjUIAAAAIAAJ Scientific memoirs, being experimental contributions to a knowledge of radiant energy]''. New York: Harper.

</div>
===Patents===
<div class="references-small">
* {{US patent|0341213}} - ''Transmitting and recording sound by radiant energy'' - Alexander Graham Bell (Filed May 3, 1884; Issued May 4, 1886.)
* {{US patent|0685957}} - ''Apparatus for the utilization of radiant energy'' - N. Tesla
* {{US patent|0685958}} - ''Method of utilizing of radiant energy'' - N. Tesla
* {{US patent|0710122}} - ''Wireless Telegraphy System'' - [[Harry Shoemaker]] (Filed Jan 11, 1902; Issued Sep 30, 1902
* {{US patent|1005338}} - ''Transmitting apparatus'' - [[Harry Shoemaker]] (Filed Jun 17, 1905; Issued Oct 10, 1911)
* {{US patent|1018555}} - ''Signaling by electroradiant energy'' - C. D. Ehret (Filed Dec 2, 1903)
* {{US patent|1317883}} - ''Method of generating radiant energy and projecting same through free air for producing heat'' - William M. Meacham (Filed Apr 12, 1915; Issued Oct 7, 1915.)
* {{US patent|1379166}} - ''Radiant energy signalling system case'' - Theodore Case (Filed Jan 22, 1918; Issued May 24, 1921.)20, 1925.) 
* {{US patent|1418792}} - ''System for control of moving bodies by radiant energy'' - [[John Hays Hammond Jr.]] (Filed Aug 6, 1914; Issued Jun 6, 1922)
* {{US patent|1425523}} - ''Transmission system for radiant energy'' - John Hays Hammond Jr. (Filed Jun 22, 1917; Issued Aug 15, 1922)
* {{US patent|1424641}} - ''Marine trailer for radiant energy receiving systems'' - John Hays Hammond Jr. (Filed Dec 23, 1918; Issued Aug 1, 1922)
* {{US patent|1523798}} - ''Perception of radiant energy'' - E. Benson (Filed Apr 13, 1918; Issued Jan 
* {{US patent|1597901}} - ''Radio apparatus'' - [[Arthur Atwater Kent]] (Filed Nov 29, 1922; Issued Aug 31, 1926.)
* {{US patent|2298272}} - ''Electromagnetic horn'' - W. L. Barrow (Filed Sep 19, 1938; Issued Oct 13, 1942)
* {{US patent|2425102}} - ''Radiant energy receiver'' - Gilbert C. Larson (Filed Sep 20, 1943; Issued Sep 10, 1846.)
* {{US patent|3971938}} - ''Method of generating electricity from radiant energy called variable polarizability capacity generator'' - L. R. O'Hare
* {{US patent|7053576}} - ''Energy conversion systems'' - Paulo N. Correa and Alexandra N. Correa (Filed Oct 15, 2002; Issued May 30, 2006)

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<!--Categories-->
[[Category:Electromagnetic radiation]]
[[Category:Radiometry]]

[[ca:Energia radiant]]
[[cs:Energie záření]]
[[fr:Énergie électromagnétique]]
[[it:Grandezze radiometriche]]
[[ja:放射エネルギー]]
[[pt:Energia radiante]]
[[sk:Žiarivá energia]]
[[tr:Işınım erkesi]]

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