A light guide (11; 101; 111) comprising first and second oppositely arranged faces, an in-coupling portion (13a-f) for in-coupling of light from a light-source (12a-f; 95a-f; 102; 106a-c; 112a-f), and an out-coupling portion (15a-f; 103; 113a-f) located adjacent to the in-coupling portion (13a-f). The out-coupling portion (15a-f; 103; 113a-f) is configured to out-couple a primary light beam having a direction of propagation directed from a position in the in-coupling portion (13a-f) with a lower out-coupling efficiency than a secondary light beam having a direction of propagation directed from a position in the light guide (11; 101; 111) outside the in-coupling portion (13a-f). In this manner, a good mixing of light in the light guide can be achieved without imposing any particular requirements on the collimation of the in-coupled light.