The invention relates to a lighting unit (1) having a concave reflector (2) with an axis of symmetry (3) and a light emission window 21) bounded by a circumferential edge (20) transverse to the axis, an elongate light source (30) extending substantially along the axis of symmetry (3), which light source (30) is accommodated in a holder (4) opposite the light emission window (21), and an axially positioned cap (5), which cap partly surrounds the light source (30) and forms an optical screening means to intercept unreflected light rays. According to the invention, the cap (5) forms part of a sleeve (60) surrounding the light source (30).