A beam of light stimulates the retina weakly when its entry to the pupil is gradually shifted from the centre toward the
edge. For single pupil entrance point the light, irrespective of its coherence would still show the Stiles-Crawford effect
with diminished visibility toward the edge of the pupil. Only when coherent light is incident from opposing points in the
pupil can the effect be cancelled. This paper has attempted a theoretical computation of how the contrast in an
interference pattern formed on the retina controls the retina’s response in three ways; first, by completely disregarding
the Stiles Crawford diminution of effective brightness for unit contrast; next, taking the traditional SCE route for zero
contrast, and finally enhancing the diminution in the effective brightness by giving an opposing boost to the visibility in
commensurate with a contrast intermediate between the two extremes of unity and zero.