In radiometry, radiant exitance or radiant emittance is the radiant flux emitted by a surface per unit area, whereas spectral exitance or spectral emittance is the radiant exitance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. This is the emitted component of radiosity. The SI unit of radiant exitance is the watt per square metre (W/m^{2}), while that of spectral exitance in frequency is the watt per square metre per hertz (W·m^{−2}·Hz^{−1}) and that of spectral exitance in wavelength is the watt per square metre per metre (W·m^{−3})—commonly the watt per square metre per nanometre (W·m^{−2}·nm^{−1}). The CGS unit erg per square centimeter per second (erg·cm^{−2}·s^{−1}) is often used in astronomy. Radiant exitance is often called "intensity" in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity.
Radiant exitance of a surface, denoted M_{e} ("e" for "energetic", to avoid confusion with photometric quantities), is defined as^{[1]}
If we want to talk about the radiant flux received by a surface, we speak of irradiance.
The radiant exitance of a black surface, according to the Stefan–Boltzmann law, is equal to:
Spectral exitance in frequency of a surface, denoted M_{e,ν}, is defined as^{[1]}
where ν is the frequency.
Spectral exitance in wavelength of a surface, denoted M_{e,λ}, is defined as^{[1]}
The spectral exitance of a black surface around a given frequency or wavelength, according to the Lambert's cosine law and the Planck's law, is equal to:
where h is the Planck constant, ν is the frequency, λ is the wavelength, k is the Boltzmann constant, c is the speed of light in the medium, T is the temperature of that surface. For a real surface, the spectral exitance is equal to:
Quantity  Unit  Dimension  Notes  

Name  Symbol^{[nb 1]}  Name  Symbol  Symbol  
Radiant energy  Q_{e}^{[nb 2]}  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}^{[nb 2]}  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}^{[nb 3]}  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}.  
Φ_{e,λ}^{[nb 4]}  watt per metre  W/m  M⋅L⋅T^{ −3}  
Radiant intensity  I_{e,Ω}^{[nb 5]}  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}^{[nb 3]}  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.  
I_{e,Ω,λ}^{[nb 4]}  watt per steradian per metre  W⋅sr^{−1}⋅m^{−1}  M⋅L⋅T^{ −3}  
Radiance  L_{e,Ω}^{[nb 5]}  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}^{[nb 3]}  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".  
L_{e,Ω,λ}^{[nb 4]}  watt per steradian per square metre, per metre  W⋅sr^{−1}⋅m^{−3}  M⋅L^{−1}⋅T^{ −3}  
Irradiance Flux density 
E_{e}^{[nb 2]}  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}^{[nb 3]}  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". NonSI 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).  
E_{e,λ}^{[nb 4]}  watt per square metre, per metre  W/m^{3}  M⋅L^{−1}⋅T^{ −3}  
Radiosity  J_{e}^{[nb 2]}  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}^{[nb 3]}  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".  
J_{e,λ}^{[nb 4]}  watt per square metre, per metre  W/m^{3}  M⋅L^{−1}⋅T^{ −3}  
Radiant exitance  M_{e}^{[nb 2]}  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}^{[nb 3]}  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".  
M_{e,λ}^{[nb 4]}  watt per square metre, per metre  W/m^{3}  M⋅L^{−1}⋅T^{ −3}  
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}^{[nb 3]}  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".  
H_{e,λ}^{[nb 4]}  joule per square metre, per metre  J/m^{3}  M⋅L^{−1}⋅T^{ −2}  
