title: Radiative flux
text: # Radiative flux
  
  Radiative flux, also known as radiative flux density or radiation flux (or sometimes power flux density), is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in $W/m^2.$ It is used in astronomy to determine the magnitude and spectral class of a star and in meteorology to determine the intensity of the convection in the planetary boundary layer. Radiative flux also acts as a generalization of heat flux, which is equal to the radiative flux when restricted to the infrared spectrum.
  
  When radiative flux is incident on a surface, it is often called irradiance. Flux emitted from a surface may be called radiant exitance or radiant emittance. The ratio of irradiance reflected to the irradiance received by a surface is called albedo.
  
  ## Shortwave radiation flux
  
  Shortwave flux is a result of specular and diffuse reflection of incident shortwave radiation by the underlying surface. This shortwave radiation, as solar radiation, can have a profound impact on certain biophysical processes of vegetation, such as canopy photosynthesis and land surface energy budgets, by being absorbed into the soil and canopies. As it is the main energy source of most weather phenomena, the solar shortwave radiation is used extensively in numerical weather prediction.
  
  ## Longwave radiation flux
  
  Longwave flux is a product of both downwelling infrared energy as well as emission by the underlying surface. The cooling associated with the divergence of longwave radiation is necessary for creating and sustaining lasting inversion layers close to the surface during polar night. Longwave radiation flux divergence also plays a role in the formation of fog.
  
  ## SI radiometry units
  
  SI radiometry unitsvte :
  
  | Quantity | Quantity | Unit | Unit | Dimension | Notes |
  | - | - | - | - | - | - |
  | Name | Symbol | Name | Symbol | Symbol | Notes |
  | Radiant energy | $Q_e$ | joule | J | $M⋅L^2⋅T^{−2}$ | Energy of electromagnetic radiation. |
  | Radiant energy density | $w_e$ | joule per cubic metre | $J/m^3$ | $M⋅L^{−1}⋅T^{−2}$ | Radiant energy per unit volume. |
  | Radiant flux | $Φ_e$ | watt | W = J/s | $M⋅L^2⋅T^{−3}$ | Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power", and called luminosity in Astronomy. |
  | Spectral flux | $Φ_{e,ν}$ | watt per hertz | W/Hz | $M⋅L^2⋅T^{−2}$ | Radiant flux per unit frequency or wavelength. The latter is commonly measured in $W⋅nm^{−1}.$ |
  | Spectral flux | $Φ_{e,λ}$ | watt per metre | W/m | $M⋅L⋅T^{−3}$ | Radiant flux per unit frequency or wavelength. The latter is commonly measured in $W⋅nm^{−1}.$ |
  | Radiant intensity | $I_{e,Ω}$ | watt per steradian | W/sr | $M⋅L^2⋅T^{−3}$ | Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity. |
  | Spectral intensity | $I_{e,Ω,ν}$ | watt per steradian per hertz | $W⋅sr^{−1}⋅Hz^{−1}$ | $M⋅L^2⋅T^{−2}$ | Radiant intensity per unit frequency or wavelength. The latter is commonly measured in $W⋅sr^{−1}⋅nm^{−1}.$ This is a directional quantity. |
  | Spectral intensity | $I_{e,Ω,λ}$ | watt per steradian per metre | $W⋅sr^{−1}⋅m^{−1}$ | $M⋅L⋅T^{−3}$ | Radiant intensity per unit frequency or wavelength. The latter is commonly measured in $W⋅sr^{−1}⋅nm^{−1}.$ This is a directional quantity. |
  | Radiance | $L_{e,Ω}$ | watt per steradian per square metre | $W⋅sr^{−1}⋅m^{−2}$ | $M⋅T^{−3}$ | Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity". |
  | Spectral radiance Specific intensity | $L_{e,Ω,ν}$ | watt per steradian per square metre per hertz | $W⋅sr^{−1}⋅m^{−2}⋅Hz^{−1}$ | $M⋅T^{−2}$ | Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in $W⋅sr^{−1}⋅m^{−2}⋅nm^{−1}.$ This is a directional quantity. This is sometimes also confusingly called "spectral intensity". |
  | Spectral radiance Specific intensity | $L_{e,Ω,λ}$ | watt per steradian per square metre, per metre | $W⋅sr^{−1}⋅m^{−3}$ | $M⋅L^{−1}⋅T^{−3}$ | Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in $W⋅sr^{−1}⋅m^{−2}⋅nm^{−1}.$ This is a directional quantity. This is sometimes also confusingly called "spectral intensity". |
  | Irradiance Flux density | $E_e$ | watt per square metre | $W/m^2$ | $M⋅T^{−3}$ | Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity". |
  | Spectral irradiance Spectral flux density | $E_{e,ν}$ | watt per square metre per hertz | $W⋅m^{−2}⋅Hz^{−1}$ | $M⋅T^{−2}$ | Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (1 Jy = $10^{−26}$ $W⋅m^{−2}⋅Hz^{−1})$ and solar flux unit (1 sfu = $10^{−22}$ $W⋅m^{−2}⋅Hz^{−1}$ = $10^4$ Jy). |
  | Spectral irradiance Spectral flux density | $E_{e,λ}$ | watt per square metre, per metre | $W/m^3$ | $M⋅L^{−1}⋅T^{−3}$ | Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (1 Jy = $10^{−26}$ $W⋅m^{−2}⋅Hz^{−1})$ and solar flux unit (1 sfu = $10^{−22}$ $W⋅m^{−2}⋅Hz^{−1}$ = $10^4$ Jy). |
  | Radiosity | $J_e$ | watt per square metre | $W/m^2$ | $M⋅T^{−3}$ | Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity". |
  | Spectral radiosity | $J_{e,ν}$ | watt per square metre per hertz | $W⋅m^{−2}⋅Hz^{−1}$ | $M⋅T^{−2}$ | Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in $W⋅m^{−2}⋅nm^{−1}.$ This is sometimes also confusingly called "spectral intensity". |
  | Spectral radiosity | $J_{e,λ}$ | watt per square metre, per metre | $W/m^3$ | $M⋅L^{−1}⋅T^{−3}$ | Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in $W⋅m^{−2}⋅nm^{−1}.$ This is sometimes also confusingly called "spectral intensity". |
  | Radiant exitance | $M_e$ | watt per square metre | $W/m^2$ | $M⋅T^{−3}$ | Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity". |
  | Spectral exitance | $M_{e,ν}$ | watt per square metre per hertz | $W⋅m^{−2}⋅Hz^{−1}$ | $M⋅T^{−2}$ | Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in $W⋅m^{−2}⋅nm^{−1}.$ "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity". |
  | Spectral exitance | $M_{e,λ}$ | watt per square metre, per metre | $W/m^3$ | $M⋅L^{−1}⋅T^{−3}$ | Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in $W⋅m^{−2}⋅nm^{−1}.$ "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity". |
  | Radiant exposure | $H_e$ | joule per square metre | $J/m^2$ | $M⋅T^{−2}$ | Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence". |
  | Spectral exposure | $H_{e,ν}$ | joule per square metre per hertz | $J⋅m^{−2}⋅Hz^{−1}$ | $M⋅T^{−1}$ | Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in $J⋅m^{−2}⋅nm^{−1}.$ This is sometimes also called "spectral fluence". |
  | Spectral exposure | $H_{e,λ}$ | joule per square metre, per metre | $J/m^3$ | $M⋅L^{−1}⋅T^{−2}$ | Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in $J⋅m^{−2}⋅nm^{−1}.$ This is sometimes also called "spectral fluence". |
  | See also: SIRadiometryPhotometry ||||||
  
  * ↑ Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
  * 1 2 3 4 5 Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant exitance.
  * 1 2 3 4 5 6 7 Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek letter nu, not to be confused with a letter "v", indicating a photometric quantity.)
  * 1 2 3 4 5 6 7 Spectral quantities given per unit wavelength are denoted with suffix "λ".
  * 1 2 Directional quantities are denoted with suffix "Ω".
  