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In radiometry, radiant exitance is the radiant flux emitted by a surface per unit area, and spectral exitance 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 emittance" is an old term for this quantity. 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]}
where
If we want to talk about the radiant flux received by a surface, we speak of irradiance.
The radiance exitance of a black surface, according to the Stefan–Boltzmann law, is equal to:
so for a real surface, the radiant exitance is equal to:
where ε is the emissivity of that surface.
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]}
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:
so for a real surface, the spectral exitance is equal to:
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