optical throughput

of a spectrometer
symbol: $G$
synonym: étendue
https://doi.org/10.1351/goldbook.08277
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 [Terms] [Paper]