The
pupillary light reflex (
PLR) or
photopupillary reflex is a
reflex that controls the diameter of the
pupil, in response to the intensity (
luminance) of light that falls on the
retinal ganglion cells of the
eye, thereby assisting in
adaptation to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (
miosis/myosis) (allowing less light in), whereas a lower intensity of light causes the pupil to dilate (
mydriasis, expansion) (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.