Volume in phase space, a function of the area of the emitting source (
\(S\)) and the solid angle into which it propagates
\((\mathit{\Omega}).{\rm{d}}^{2} G = {\rm{d}}S {\rm{d}}\mathit{\Omega}\).
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 669 (https://doi.org/10.1515/pac-2019-0203)