Abstract: An optical system forms an image of a light source, such as a short arc lamp, particularly of a high numerical aperture, via a spherical, concave reflector onto the entrance surface of a light guide. The reflector is asymmetrically arranged to the light source, and the latter has a relative large angular distance from the axis of symmetry of the reflector, so that the light guide in the path of reflected rays has also a large angular distance from this axis of symmetry and, hence, from the hot light source. In the path of rays between the light source and the spherical, concave reflector is a cylindrical lens, preferably a circular cylindrical lens, of positive diffraction.