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.
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Radiant exitance
Radiant exitance of a surface, denoted Me ("e" for "energetic", to avoid confusion with photometric quantities), is defined as
where
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:
where
so for a real surface, the radiant exitance is equal to:
where ε is the emissivity of that surface.
Spectral exitance
Spectral exitance in frequency of a surface, denoted Me,ν, is defined as
where ν is the frequency.
Spectral exitance in wavelength of a surface, denoted Me,λ, is defined as
where λ is the wavelength.
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
so for a real surface, the spectral exitance is equal to: