Illumination device comprising a collimator

An illumination device arranged to generate illumination device light, the illumination device comprising a lighting unit, the lighting unit comprising:

• a light source, arranged to generate light source light, and
• a substantially flat collimator, arranged to collimate the light source light, the collimator having:
• an entrance window and an edge window,
• a top collimator surface and a bottom collimator surface, which are arranged substantially parallel,
• a first collimating side edge and a second collimating side edge, which are arranged to diverge from the entrance window to the edge window, and
• an optical axis (O),
• wherein the entrance window is arranged to receive the light source light of the light source, and wherein the edge window is arranged to allow the collimated light source light escape from the collimator as illumination device light;
• wherein one or more of the top collimator surface, the bottom collimator surface, the first collimating side edge and the second collimating side edge comprise one or more grooves, each having a longitudinal axis having a groove direction angle (ω) with the optical axis (O) ≧0° and <90°.