In radiometry, radiosity is the radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area, and spectral radiosity is the radiosity of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength.^{[1]} The SI unit of radiosity is the watt per square metre (W/m^{2}), while that of spectral radiosity in frequency is the watt per square metre per hertz (W·m^{−2}·Hz^{−1}) and that of spectral radiosity 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. Radiosity is often called intensity^{[2]} in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity.
Radiosity  

Common symbols  
SI unit  W·m^{−2} 
Other units  erg·cm^{−2}·s^{−1} 
Dimension  M T^{−3} 
Radiosity of a surface, denoted J_{e} ("e" for "energetic", to avoid confusion with photometric quantities), is defined as^{[3]}
where
For an opaque surface, the transmitted component of radiosity J_{e,tr} vanishes and only two components remain:
In heat transfer, combining these two factors into one radiosity term helps in determining the net energy exchange between multiple surfaces.
Spectral radiosity in frequency of a surface, denoted J_{e,ν}, is defined as^{[3]}
where ν is the frequency.
Spectral radiosity in wavelength of a surface, denoted J_{e,λ}, is defined as^{[3]}
where λ is the wavelength.
The radiosity of an opaque, gray and diffuse surface is given by
where
Normally, E_{e} is the unknown variable and will depend on the surrounding surfaces. So, if some surface i is being hit by radiation from some other surface j, then the radiation energy incident on surface i is E_{e,ji} A_{i} = F_{ji} A_{j} J_{e,j} where F_{ji} is the view factor or shape factor, from surface j to surface i. So, the irradiance of surface i is the sum of radiation energy from all other surfaces per unit surface of area A_{i}:
Now, employing the reciprocity relation for view factors F_{ji} A_{j} = F_{ij} A_{i},
and substituting the irradiance into the equation for radiosity, produces
For an N surface enclosure, this summation for each surface will generate N linear equations with N unknown radiosities,^{[4]} and N unknown temperatures. For an enclosure with only a few surfaces, this can be done by hand. But, for a room with many surfaces, linear algebra and a computer are necessary.
Once the radiosities have been calculated, the net heat transfer at a surface can be determined by finding the difference between the incoming and outgoing energy:
Using the equation for radiosity J_{e,i} = ε_{i}σT_{i}^{4} + (1 − ε_{i})E_{e,i}, the irradiance can be eliminated from the above to obtain
where M_{e,i}° is the radiant exitance of a black body.
For an enclosure consisting of only a few surfaces, it is often easier to represent the system with an analogous circuit rather than solve the set of linear radiosity equations. To do this, the heat transfer at each surface is expressed as
where R_{i} = (1 − ε_{i})/(A_{i}ε_{i}) is the resistance of the surface.
Likewise, M_{e,i}^{°} − J_{e,i} is the blackbody exitance minus the radiosity and serves as the 'potential difference'. These quantities are formulated to resemble those from an electrical circuit V = IR.
Now performing a similar analysis for the heat transfer from surface i to surface j,
where R_{ij} = 1/(A_{i} F_{ij}).
Because the above is between surfaces, R_{ij} is the resistance of the space between the surfaces and J_{e,i} − J_{e,j} serves as the potential difference.
Combining the surface elements and space elements, a circuit is formed. The heat transfer is found by using the appropriate potential difference and equivalent resistances, similar to the process used in analyzing electrical circuits.
In the radiosity method and circuit analogy, several assumptions were made to simplify the model. The most significant is that the surface is a diffuse emitter. In such a case, the radiosity does not depend on the angle of incidence of reflecting radiation and this information is lost on a diffuse surface. In reality, however, the radiosity will have a specular component from the reflected radiation. So, the heat transfer between two surfaces relies on both the view factor and the angle of reflected radiation.
It was also assumed that the surface is a gray body, that is to say its emissivity is independent of radiation frequency or wavelength. However, if the range of radiation spectrum is large, this will not be the case. In such an application, the radiosity must be calculated spectrally and then integrated over the range of radiation spectrum.
Yet another assumption is that the surface is isothermal. If it is not, then the radiosity will vary as a function of position along the surface. However, this problem is solved by simply subdividing the surface into smaller elements until the desired accuracy is obtained.^{[4]}
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}  
