Radiant energy

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Visible light (a form of electromagnetic radiation carrying radiant energy) scattered by fog in a forest.

Radiant energy is the energy of electromagnetic radiation.[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. The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.[2][3]

Terminology use and history[edit]

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). 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.[4]


Cherenkov radiation glowing in the core of a TRIGA reactor.

Because electromagnetic (EM) radiation can be conceptualized 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 converted to heat (or converted to electricity in case of a photoelectric material). 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[edit]

Radiant energy is one of the mechanisms by which energy can enter or leave an open system.[5][6][7] Such a system can be man-made, such as a solar energy collector, or natural, such as the Earth's atmosphere. In geophysics, most atmospheric gases, including the greenhouse gases, allow the Sun's short-wavelength radiant energy to pass through to the Earth's surface, heating the ground and oceans. The absorbed solar energy is partly re-emitted as longer wavelength radiation (chiefly infrared radiation), some of which is absorbed by the atmospheric greenhouse gases. Radiant energy is produced in the sun as a result of nuclear fusion.[8]


Radiant energy is used for radiant heating.[9] 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. Because of this, the air temperature may be lower than in a conventionally heated building, even though the room appears just as comfortable.

Various other applications of radiant energy have been devised.[10] These include:

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 current flow or some other perceivable change, such as exposure of photographic film.

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.[11] 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[edit]

SI radiometry units
NameSymbol[nb 1]NameSymbolSymbol
Radiant energyQe[nb 2]jouleJML2T−2energy
Radiant fluxΦe[nb 2]wattW or J/sML2T−3radiant energy per unit time, also called radiant power.
Spectral powerΦ[nb 2][nb 3]watt per metreW⋅m−1MLT−3radiant power per wavelength.
Radiant intensityIewatt per steradianW⋅sr−1ML2T−3power per unit solid angle.
Spectral intensityI[nb 3]watt per steradian per metreW⋅sr−1⋅m−1MLT−3radiant intensity per wavelength.
RadianceLewatt per steradian per square metreW⋅sr−1m−2MT−3power per unit solid angle per unit projected source area.

confusingly called "intensity" in some other fields of study.

Spectral radianceL[nb 3]
L[nb 4]
watt per steradian per metre3

watt per steradian per square
metre per hertz

commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.

IrradianceEe[nb 2]watt per square metreW⋅m−2MT−3power incident on a surface, also called radiant flux density.

sometimes confusingly called "intensity" as well.

Spectral irradianceE[nb 3]
E[nb 4]
watt per metre3
watt per square metre per hertz
commonly measured in W⋅m−2nm−1
or 10−22 W⋅m−2⋅Hz−1, known as solar flux unit.[nb 5]

Radiant exitance /
Radiant emittance
Me[nb 2]watt per square metreW⋅m−2MT−3power emitted from a surface.
Spectral radiant exitance /
Spectral radiant emittance
M[nb 3]
M[nb 4]
watt per metre3

watt per square
metre per hertz

power emitted from a surface per unit wavelength or frequency.

RadiosityJewatt per square metreW⋅m−2MT−3emitted plus reflected power leaving a surface.
Spectral radiosityJ[nb 3]watt per metre3W⋅m−3ML−1T−3emitted plus reflected power leaving a surface per unit wavelength
Radiant exposureHejoule per square metreJ⋅m−2MT−2also referred to as fluence
Radiant energy densityωejoule per metre3J⋅m−3ML−1T−2
See also: SI · Radiometry · Photometry
  1. ^ Standards organizations recommend that radiometric quantities should be denoted with a suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
  2. ^ a b c d e Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant emittance.
  3. ^ a b c d e f Spectral quantities given per unit wavelength are denoted with suffix "λ" (Greek) to indicate a spectral concentration. Spectral functions of wavelength are indicated by "(λ)" in parentheses instead, for example in spectral transmittance, reflectance and responsivity.
  4. ^ a b c Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek)—not to be confused with the suffix "v" (for "visual") indicating a photometric quantity.
  5. ^ NOAA / Space Weather Prediction Center includes a definition of the solar flux unit (SFU).

See also[edit]

Notes and references[edit]

  1. ^ "Radiant energy". Federal standard 1037C
  2. ^ George Frederick Barker, Physics: Advanced Course, page 367
  3. ^ Hardis, Jonathan E., "Visibility of Radiant Energy". PDF.
  4. ^ Examples: US 1005338  "Transmitting apparatus", US 1018555  "Signaling by electroradiant energy", and US 1597901  "Radio apparatus".
  5. ^ 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 0-471-27471-2.
  6. ^ Robert W. Christopherson, Elemental Geosystems, Fourth Edition. Prentice Hall, 2003. Pages 608. ISBN 0-13-101553-2
  7. ^ James Grier Miller and Jessie L. Miller, The Earth as a System.
  8. ^ Energy transformation. assets.cambridge.org. (excerpt)
  9. ^ US 1317883  "Method of generating radiant energy and projecting same through free air for producing heat"
  10. ^ Class 250, Radiant Energy, USPTO. March 2006.
  11. ^ 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.

Further reading[edit]

  • Caverly, Donald Philip, Primer of Electronics and Radiant Energy. New York, McGraw-Hill, 1952.
  • Whittaker, E. T. (Apr 1929). "What is energy?". The Mathematical Gazette (The Mathematical Association) 14 (200): 401–406. doi:10.2307/3606954. JSTOR 3606954.