US Patent 8475010 Illumination device comprising a collimator

The invention provides an illumination device (1) comprising a lighting unit (2). The lighting unit (2) comprises a light source (100) and a substantially flat collimator (200), arranged to collimate light source light (111). The collimator (200) has an entrance window (210), an edge window (220), a top collimator surface (201), a bottom collimator surface (202), a first collimating side edge (230) and a second collimating side edge (240). The lighting unit has an optical axis (O). One or more of the top collimator surface (201), the bottom collimator surface (202), the first collimating side edge (230) and the second collimating side edge (240) comprise n*½ grooves (300), wherein n is a positive integer number, and wherein the grooves (300) independently have a longitudinal axis (301) having a groove direction angle (ω) with the optical axis (O) ≧0° and & and <90°.