The angle, with vertex at the focus, subtended by the edges of a lens or reflector, usually measured in a horizontal or vertical plane. (Figs. 10 and 16b)
Note: In France, the subtense angle of an optical panel, measured in the horizontal plane, is called Ouverture.
In Germany, the subtense angle of a fixed lens or of an optical panel, measured in the horizontal plane, is called Spannwinkel.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
See also Total Subtense
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM