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Radiant energy

  Radiant energy is the energy of electromagnetic waves.[1] The quantity of radiant energy may be calculated by integrating radiant flux (or power) with respect to time and, like all forms of energy, its SI unit is the joule. Radiant energy is generally thought of as radiation emitted by a source into the surrounding environment. It propagates in the form of electromagnetic waves, or traveling subatomic, atomic or molecular particles. Specific forms of radiant energy include electron space discharge, visible light, and other wave types.[2] Radiant energy is exhibited in the spontaneous nuclear disintegration with emission of particulate or electromagnetic radiations.


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.[3] 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, radiant energy has been called "electro-radiant energy".[4] The term and concept of "radiant energy" has historically also been applied to electrostatics.[5][6]

Historically, to account for the propagation of radiant energy — that is, light, actinic radiation, heat, electricity, cathode rays, X-rays, alpha rays, beta rays, and gamma rays — it was postulated the existence of a medium filling all space.[7][8][9] It was thought that both electrostatic and electromagnetic strains a medium, such as a aether, and the stresses in electro-magnetic energy are at right angles both to the electrostatic stresses and to the direction of their motion or flow.[10] When the material medium transmitting the radiation is at rest, the ray, or path of the radiant energy, is the same relative to the matter as to the aether.[11] In modern times, accounting for the propagation of radiant energy through the existence of a medium filling all space is possible [12] but not necessary as the electrodynamic effects and electromagnetic waves generally, it is thought, do not require a physical transmission medium unlike mechanical waves, and so can travel through the "vacuum" of free space. Regions of the insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions.


  Because electromagnetic (EM) radiation can be conceptualized, in modern times, as a stream of photons, 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 frequencies. The bands of frequency present in a given EM signal may be sharply defined, as is seen in 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 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 reflected or 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.[13][14][15] 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 gases 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.[16]


Radiant energy, as well as convective energy and conductive energy, is used for radiant heating.[17] 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).

Various other methods and apparatus involving radiant energy have been devised.[18] All methods and apparatus using, generating, controlling or detecting radiant energy involve combinations of circuits which are closed or closable conducting paths through which, or along which, electric current can travel. In general, devices involving radiant energy is provided on the basis of either a specific use of the radiant energy or a specific type of radiant energy. Devices explicitly providing for subject matter involving radiant energy are:

  • Treatment and inspection
  • Separating and assorting
  • Medium of control
  • Medium of communication

Inspection is used to imply a source of radiant energy, and/or means to irradiate an object by said source and a detector responsive to radiation from the object to provide a signal representing some characteristic of the object. Various devices use material which when subjected to radiation for treatment or whose response to or effect on the radiation is used to indicate something about the material.

  Radiant energy detectors produce responses to incident radiant energy either as an increase or decrease in electric potential or current flow (Electric) or some other perceivable change (Nonelectric). The nonelectric change may be immediately perceived or may require development to be perceived, e.g., photographic changes. Since radiant energy is really just electromagnetic radiation under another name, it is the basis of a wide range of communication technologies using radiofrequency and microwave radiation. Radiant energy is observed by a detector, which is any material or device whose response to radiant energy is used to indicate the presence or amount of incident radiation. This is also called "Signalling Means". Some devices are detectors used to sense light incident thereon and generate a signal representative of some aspect of the light such as intensity, phase, coherence, mode distribution, and interference pattern characteristics.

One of the earliest wireless telephones 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.[19] 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


Quantity Symbol SI unit Abbr. Notes
Radiant energy Q joule J energy
Radiant flux Φ watt W radiant energy per unit time, also called radiant power
Radiant intensity I watt per steradian W·sr−1 power per unit solid angle
Radiance L watt per steradian per square metre W·sr−1·m−2 power per unit solid angle per unit projected source area.

Sometimes confusingly called "intensity".

Irradiance E watt per square metre W·m−2 power incident on a surface.

Sometimes confusingly called "intensity".

Radiant exitance / Radiant emittance M watt per square metre W·m−2 power emitted from a surface.
Radiosity J or Jλ watt per square metre W·m−2 emitted plus reflected power leaving a surface
Spectral radiance Lλ
watt per steradian per metre3 or

watt per steradian per square metre per hertz



commonly measured in W·sr−1·m−2·nm−1
Spectral irradiance Eλ
watt per metre3 or
watt per square metre per hertz
commonly measured in W·m−2·nm−1

See also

Energy Portal
Main concepts
Luminous energy, Power, Radiometry, Federal Standard 1037C, Transmission, Electrostatics, Ionizing radiation
Photoelectric effect, Zero-point energy, Open system, Cosmic microwave background radiation, Schumann resonance
Photonic devices
Photodetector, Photocell, Photoelectric cell
Radio spectrum
3 Hz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz
30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz

References and further reading


  1. ^ "Radiant energy". Federal standard 1037C
  2. ^ This would include all electrical phenomena of transverse waves (those with vibrations perpendicular to the direction of the propagation) and longitudinal waves (those with vibrations parallel to the direction of the propagation).
  3. ^ George Frederick Barker, Physics: Advanced Course. Page 367
  4. ^ Examples of the term 'electroradiant energy' include U.S. Patent 1005338, 1018555, and 1597901.
  5. ^ William H. Preece, "On the transmisson of electric signals through space", electrostatic induction, The Electrical engineer. (1884). London: Biggs & Co. Page 200
  6. ^ Thomas Preston, "The Theory of Light". Macmillan, 1901. 586 pages. Page 544.
  7. ^ Thomas Preston, "The Theory of Light". Page 542.
  8. ^ George Frederick Barker, Physics: Advanced Course. Page 365
  9. ^ Frederick Booth, Radiant Energy and the Ophthalmic Lens.
  10. ^ Bell, Louis (1901). Electric Power Transmission; a Practical Treatise for Practical Men. Electrical World and Engineer, p. 10. Retrieved on 2007-02-15. 
  11. ^ Sir Joseph Larmor, Aether and Matter. Page 30.
  12. ^ Albert Einstein stressed that space is "endowed with physical quantities" He held that general relativity attributed tangible physical properties to space including some kind of medium for light, although not a material one. He stated in the 1920 paper, "Ether and the Theory of Relativity", "[the aether is] allowed to assume a space-filling medium if one can refer to electromagnetic fields (and thus also for sure matter) as the condition thereof". Additionally, Paul Dirac, in a 1951 article in Nature, titled "Is there an ether?" that "we are rather forced to have an ether"'. Dirac wrote about his theory: "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."
  13. ^ 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.
  14. ^ Robert W. Christopherson, Elemental Geosystems, Fourth Edition. Prentice Hall, 2003. Pages 608. ISBN 0131015532
  15. ^ James Grier Miller and Jessie L. Miller, The Earth as a System.
  16. ^ Energy transformation. (excerpt)
  17. ^ U.S. Patent 1317883 - Method of generating radiant energy and projecting same through free air for producing heat
  18. ^ Class 250, Radiant Energy, USPTO. March 2006.
  19. ^ 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.

General Information

  • Patents > Guidance, Tools, and Manuals >> Classification >>> Class Definition : 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). 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., "Visibility of Radiant Energy". PDF.
  • Lighting Design Knowledgebase
  • Sir Joseph Larmor, 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, Physics: Advanced Course. Henry Holt and Company, 1893. 902 pages.
  • Frederick Booth, Radiant Energy and the Ophthalmic Lens. P. Blakiston's Son & Co., 1921. 226 pages.
  • Thomas O'Conor Sloane, Electricity Simplified: The Practice and Theory of Electricity. The N.W. Henley publishing co., 1905. 172 pages.
  • "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., What Is Energy?. The Mathematical Gazette, 1929.
  • Peter Michael Harman, George Basalla, and Owen Hannaway, "Energy, Force and Matter: The Conceptual Development of Nineteenth-century Physics". Cambridge University Press, 1982.
  • U. Fano, 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, Electro-Optics Handbook. CRC Press Technology & Industrial Arts, 1979. ISBN 0824768795
  • Draper, J. W. (1878). Scientific memoirs, being experimental contributions to a knowledge of radiant energy. New York: Harper.


  • U.S. Patent 0,341,213  - Transmitting and recording sound by radiant energy - Alexander Graham Bell (Filed May 3, 1884; Issued May 4, 1886.)
  • U.S. Patent 0,685,957  - Apparatus for the utilization of radiant energy - N. Tesla
  • U.S. Patent 0,685,958  - Method of utilizing of radiant energy - N. Tesla
  • U.S. Patent 0,710,122  - Wireless Telegraphy System - Harry Shoemaker (Filed Jan 11, 1902; Issued Sep 30, 1902
  • U.S. Patent 1,005,338  - Transmitting apparatus - Harry Shoemaker (Filed Jun 17, 1905; Issued Oct 10, 1911)
  • U.S. Patent 1,018,555  - Signaling by electroradiant energy - C. D. Ehret (Filed Dec 2, 1903)
  • U.S. Patent 1,317,883  - 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.)
  • U.S. Patent 1,379,166  - Radiant energy signalling system case - Theodore Case (Filed Jan 22, 1918; Issued May 24, 1921.)20, 1925.)
  • U.S. Patent 1,418,792  - System for control of moving bodies by radiant energy - John Hays Hammond Jr. (Filed Aug 6, 1914; Issued Jun 6, 1922)
  • U.S. Patent 1,425,523  - Transmission system for radiant energy - John Hays Hammond Jr. (Filed Jun 22, 1917; Issued Aug 15, 1922)
  • U.S. Patent 1,424,641  - Marine trailer for radiant energy receiving systems - John Hays Hammond Jr. (Filed Dec 23, 1918; Issued Aug 1, 1922)
  • U.S. Patent 1,523,798  - Perception of radiant energy - E. Benson (Filed Apr 13, 1918; Issued Jan
  • U.S. Patent 1,597,901  - Radio apparatus - Arthur Atwater Kent (Filed Nov 29, 1922; Issued Aug 31, 1926.)
  • U.S. Patent 2,298,272  - Electromagnetic horn - W. L. Barrow (Filed Sep 19, 1938; Issued Oct 13, 1942)
  • U.S. Patent 2,425,102  - Radiant energy receiver - Gilbert C. Larson (Filed Sep 20, 1943; Issued Sep 10, 1846.)
  • U.S. Patent 3,971,938  - Method of generating electricity from radiant energy called variable polarizability capacity generator - L. R. O'Hare
  • U.S. Patent 7,053,576  - Energy conversion systems - Paulo N. Correa and Alexandra N. Correa (Filed Oct 15, 2002; Issued May 30, 2006)
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Radiant_energy". A list of authors is available in Wikipedia.
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