## radiant exitance, $$M$$

@[email protected], $$P$$, emitted at all wavelengths by an element of surface containing the source point under consideration divided by the area of the source. SI unit is $$\text{W m}^{-2}$$.
1. Mathematical definition: $$M = \text{d}P/\text{d}S$$. If the @[email protected] $$P$$ is constant over the surface area considered, $$M = P/S$$.
2. Equivalent to the integration of the @[email protected] leaving a source over the solid @[email protected] and over the whole @[email protected] range. Mathematical definition: $$M = \int_{\lambda}M_{\lambda}\, \text{d}\lambda$$, where $$M_{\lambda}$$ is the @[email protected] at @[email protected] $$\lambda$$.