It is presented a light guide (1) having a polygonal shape with a plurality of sides (5), each side (5) connected by two corners (5). At least one of the corners is an in-coupling corner (4) for in-coupling of light into the light guide (1). At least one of the sides (5) has is slanting and collimates light in-coupled from an adjacent light in-coupling corner (4). The slanting side (5) can also reflect light towards a planar light emitting surface (3) for out-coupling of light therethrough. The light in-coupled in a light in-coupling corner (4) is collimated as if the light guide (1) is an equilateral triangle (2) having three slanting sides.