The invention provides a light collimator (1) comprising (a) an elongated light waveguide (100) having a waveguide longitudinal axis (101), a waveguide length (wl), a waveguide width (ww) and a waveguide height (wh); the waveguide height (wh) defined by the height between a first waveguide surface (151) and a second waveguide surface (152), the waveguide (100) having an aspect ratio of the waveguide length (wl) and the waveguide width (ww) wl/ww>1; the waveguide comprising a plurality of elongated cavities (110); each cavity (110) comprises a cavity longitudinal axis (111), a cavity length (cl), a cavity width (cw) and a cavity height (ch); each cavity having an aspect ratio of the cavity length (cl) and the cavity width (cw) cl/cw>1, wherein the cavity longitudinal axes (111) of the plurality of cavities (110) are perpendicular to the waveguide longitudinal axis (101); and (b) a diffuse reflective layer (200) adjacent to the second waveguide surface. The invention further provides a lighting unit (2) using such collimator (1).