angular subtense [α]
visual angle subtended by a source at the eye of an observer or at the point of measurement
Unit: rad, °
NOTE 1 Subtended angles are denoted by the full included angle, not the half angle.
NOTE 2 The angular subtense, α, will often be modified by incorporation of lenses and mirrors as projector optics, in which case the angular subtense of the apparent source will differ from the angular subtense of the physical source.
NOTE 3 The angular subtense of an oblong source shall be determined by the arithmetic mean of the maximum and minimum angular dimension of the source.
NOTE 4 In safety applications (IEC62471/CIES009:2006 Photobiological safety of lamp and lamp systems) the term is used as: angle subtended by an apparent source as viewed from a point in space.