1.11: Exitance M

The exitance of an extended surface is the rate at which it is radiating energy (in all directions) per unit area. The usual symbol is \(M\) and the units are \(\text{W m}^{-2}\). It is an intrinsic property of the radiating surface and is not dependent on the position of an observer.

Most readers will be aware that some property of a black body is equal to \(\sigma T^4\). Technically it is the exitance (integrated over all wavelengths, with no subscript on the \(M\)) that is equal to \(\sigma T^4\), so that, in our notation, the Stefan-Boltzmann law would be written

\[M=\sigma T^4 \tag{1.11.1}, \label{1.11.1} \]

where \(\sigma\) has the value \(5.7 \times 10^{-8} \text{W m}^{-2} \text{K}^{-4}\).

Likewise the familiar Planck equation for a black body:

\[M_\lambda=\frac{2\pi hc^2}{\lambda^5 \left( e^{hc/kT}-1 \right)} \tag{1.11.2} \label{1.11.2} \]

gives the exitance per unit wavelength interval.

The word "emittance" is an older word for what is now called exitance.

The emissivity of a radiating surface is the ratio of its exitance at a given wavelength and temperature to the exitance of a black body at that wavelength and temperature.

---