An optical element including a unitary, non-circularly-symmetrical, piece of optically-transmissive material, which has at least first and second surfaces for concentrating light from a light source onto a linear target region, such that at least one of the first and second surfaces is curved, and such that a first portion of the light is concentrated onto the linear target region by reflection from the first surface, while a second portion of the light is concentrated onto the linear target region by refraction at the second surface.