The device comprises an aperture stop disc divided into a plurality of mutually exclusive filtering segments comprising a first, a second and a third filtering segment ; the third filtering segment is adapted to pass a third portion of the spectrum which is included in the portion of the spectrum passing the first and second filtering segments. Due to third filtering segment, the aperture assigned to the color component corresponding to the third portion of the spectrum—preferably blue—is larger than the aperture assigned to the color component corresponding to the portion of the spectrum passing the first and second filtering segments.