The angle between the forward direction of the incident beam and a straight line connecting the scattering point and the detector. Synonymous with angle of observation.
Sources:
Purple Book, 1st ed., p. 65 (http://old.iupac.org/publications/books/author/metanomski.html)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2212 (https://doi.org/10.1351/pac199062112167)