Spectral radiant exitance
= Spectral radiant exitance
{title2=$M_\nu=\frac c4U_\nu$}
Radiant <power> leaving a surface per surface <area> and per unit <frequency> or <wavelength>. Isotropic cavity radiation obeys $M_\nu=cU_\nu/4$; the factor $1/4$ combines the outward hemisphere and projected area.