# Radiant flux

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In radiometry, radiant flux or radiant power is the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light). The power may be the total emitted from a source, or the total landing on a particular surface.

## Units

The SI unit of radiant flux is the watt (W), which has dimensions of energy/time or, in SI units joules/second.

SI radiometry units
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol
Radiant energy Qe[nb 2] joule J M⋅L2⋅T−2 energy
Radiant flux Φe[nb 2] watt W M⋅L2⋅T−3 radiant energy per unit time, also called radiant power.
Spectral power Φ[nb 2][nb 3] watt per metre W⋅m−1 M⋅L⋅T−3 radiant power per wavelength.
Radiant intensity Ie watt per steradian W⋅sr−1 M⋅L2⋅T−3 power per unit solid angle.
Spectral intensity I[nb 3] watt per steradian per metre W⋅sr−1⋅m−1 M⋅L⋅T−3 radiant intensity per wavelength.
Radiance Le watt per steradian per square metre W⋅sr−1m−2 M⋅T−3 power per unit solid angle per unit projected source area.

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

Spectral radiance L[nb 3]
or
L[nb 4]
watt per steradian per metre3
or

watt per steradian per square
metre per hertz

W⋅sr−1m−3
or
W⋅sr−1⋅m−2Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.

Irradiance Ee[nb 2] watt per square metre W⋅m−2 M⋅T−3 power incident on a surface, also called radiant flux density.

sometimes confusingly called "intensity" as well.

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

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

watt per square
metre per hertz

W⋅m−3
or
W⋅m−2⋅Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
power emitted from a surface per wavelength or frequency.

Radiosity Je or J[nb 3] watt per square metre W⋅m−2 M⋅T−3 emitted plus reflected power leaving a surface.
Radiant exposure He joule per square metre J⋅m−2 M⋅T−2
Radiant energy density ωe joule per metre3 J⋅m−3 M⋅L−1⋅T−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. Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant emittance.
3. 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).

## References

Boyd, Robert (1983). Radiometry and the Detection of Optical Radiation (Pure & Applied Optics Series). Wiley-Interscience. ISBN 978-0-471-86188-1.