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/m2), 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.
Mathematical definitions
Radiant exitance
Radiant exitance of a surface, denoted Me ("e" for "energetic", to avoid confusion with photometric quantities), is defined as[1]where∂ is the partial derivative symbol,Φe is the radiant fluxemitted, andA is the surface area.
If we want to talk about the radiant flux received by a surface, we speak of irradiance.
whereh 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:
SI radiometry units
^Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
^ a b c d eAlternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant exitance.
^ a b c d e f gSpectral 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.)
^ a b c d e f gSpectral quantities given per unit wavelength are denoted with suffix "λ".
^ a bDirectional quantities are denoted with suffix "Ω".