Specification of the Directionality of the Stiles‐Crawford Function

An index to specify the directionality (spread) of Stiles‐Crawford function and related retinal photoreceptor directional sensitivity data is proposed. This index, termed the half‐sensitivity half‐width (or angle) is the displacement in the pupil (or the change of angle of incidence at the retina) which results in a reduction of sensitivity to one‐half that at the function maximum (0.3 log unit decrease). The half‐sensitivity half‐width or angle, unlike other currently used indices of retinal directionality, can be assessed empirically and is not intrinsically dependent upon curve‐fitting assumptions or procedures.