The invention provides a substantially round illumination device (1) comprising a light source (10) and a substantially round waveguide (20). The waveguide (20) comprises a first waveguide surface (21), a second waveguide surface (22), a substantially round waveguide entrance window (23), a substantially round waveguide edge window (24), and a central axis (100). The first waveguide surface (21) or the second waveguide surface (22) or both the first waveguide surface (21) and the second waveguide surface (22) further comprise a plurality of elongated structures (200) each having an elongation axis (201) substantially parallel to a radius (101) perpendicular to the central axis (100).