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Radiant Exitance

Definition: radiant flux emitted by a surface per unit area

Alternative term: radiant emittance

German: Ausstrahlung

Category: light detection and characterization

Formula symbol: Me

Units: W/m2, W/cm2

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radiant exitance
Figure 1: Radiant exitance is emitted radiant flux per unit source area.

Radiant exitance (or emittance) is a term of radiometry and is defined as the radiant flux (optical power = energy per unit time) which is emitted by some surface (e.g. of a light source) per unit area. For example, if an optical power of 1 μW is radiated from an area of 1 m2, the radiant excitance is 1 μW / 1 mm2 = 1 W/m2.

In the SI system, the units of the radiant exitance are W/m2 (watts per square meter). The related term irradiance has the same units, but applies to received radiation.

A related quantity is the spectral exitance, which is the exitance per unit frequency or wavelength interval. It has units of W / (m2 Hz) or W / (m2 nm), for example.

The corresponding photometric quantity is the luminous exitance.

Example: Radiant Exitance of Blackbody Radiation

A prominent example is thermal radiation from a black body at temperature T, which according to Planck's law has a spectral radiance

Planck's law

from which one can calculate the spectral exitance by integration over all solid angles of a hemisphere:

spectral exitance of blackbody radiation

If this is integrated over all frequencies, one obtains the Stefan–Boltzmann law for the radiant exitance of thermal radiation of a black body:

Stefan-Boltzmann law

with the Stefan–Boltzmann constant σ ≈ 5.6704 · 10−8 W m−2 K−4.

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See also: radiometry, irradiance
and other articles in the category light detection and characterization


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