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Radiant energy is used in radiometry.

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Radiant energy is the energy of electromagnetic waves. Radiant energy is most commonly used in the fields of radiometry, solar energy, heating and lighting, but is also used less frequently in other fields (such as telecommunications). 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. In 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).

Radiant energy analysis.

Radiant energy.
Radiant energy is the energy of electromagnetic waves.

Because electromagnetic (EM) radiation can be considered to be 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.

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 of radiant energy.

Radiant energy is one of the energy sources that can be used to power an open system. Such an open 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.

Applications and patents of radiant energy.

Radiant energy, as well as convective and conductive energy, is used for heating homes. It can be generated electrically by Infrared lamps, or can be absorbed from sunlight and used to heat water. 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.

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 (see Anderson's book, below) an experiment he had done in 1896. 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."

The United States Patent Offices has a classification of radiant energy for patent applications (Class 250, a residual class for methods and apparatus involving radiant energy). This class provides for all methods and apparatus for using, generating, controlling or detecting radiant energy, combinations including such methods or apparatus, subcombinations of same and accessories therefore not classifiable elsewhere by the patent office.

SI radiometry units of radiant energy.

SI radiometry units
QuantitySymbolSI unitAbbr.Notes
Radiant energy Q joule J energy
radiant flux F watt W radiant energy per unit time, also called radiant power
Radiant intensity I watt per steradian W新r-1 power per unit solid angle
Radiance L watt per steradian per square metre W新r-1-2 power per unit solid angle per unit projected source area.

Sometimes confusingly called "intensity".

irradiance E watt per square metre W搶-2 power incident on a surface.

Sometimes confusingly called "Intensity".

Radiant exitance / Radiant emittance M watt per square metre W搶-2 power emitted from a surface.

Sometimes confusingly called "Intensity".

Spectral radiance L?
watt per steradian per metre3 or

watt per steradian per square metre per hertz



commonly measured in W新r-1-2搖m-1
Spectral irradiance E?
watt per metre3 or
watt per square metre per hertz
commonly measured in W搶-2搖m-1


  • shortwave radiation.
  • Longwave radiation.
  • ionizing radiation.
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

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