Device and method that allows an objective measure to be taken of the size and morphological pattern of the degradation of the image in scotopic vision. It consists of a black front screen (1) which has a central luminous source (2) surrounded by a grid of circular light spots (3) which at the beginning of the test light up sequentially or at random, according to the method selected, from the nearest to the central source of glare in a centrifuge fashion to the outermost, and a trigger which the patient will press every time he can discriminate the presence of each of the spots in 12 meridians. The non-discriminated or hidden spots will border an area which is interpreted as being occupied by the light aberration and is used as a basis for calculating the glare index.