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.