The invention relates to a lighting unit having a concave reflector with an axis of symmetry and a light emission window bounded by a circumferential edge transverse to the axis, an elongate light source extending substantially along the axis of symmetry, which light source is accommodated in a holder opposite the light emission window, and an axially positioned cap, which cap partly surrounds the light source and forms an optical means to intercept unreflected light rays. According to the invention, the light source is surrounded by a sleeve with an end at the side of the light emission window, over which end the cap is positioned by way of a fixing element.