A luminaire has side reflectors having edges defining the width W of a light-emission window, and a plurality of lamellae, which have a concave outer edge in the light-emission window and an inner face remote from that window. In the centers of the lamellae, the lamellae have a distance h.sub.o between the outer edge and the inner face that is <0.1 W. As a result, the total surface area of the inner face is relatively small. Internal reflections are thereby reduced, and a higher light output is obtained. If the lamellae are solid and made of plastic, less material is required for their fabrication. In a preferred embodiment, h.sub.o<0.05 W, and the inner face may then be convex and even have the same contour as the outer edge. The lamellae louver has lamellae of h.sub.o