Abstract: An illumination system (3) comprises an optical waveguide (9) of an optically transparent material having an exit surface (19) and four end faces (13, 14, 15, 16). The light from a light source (11) is coupled into the optical waveguide (9) via at least one of the end faces (13). The illumination system, for an LCD, further has a reflective polarizer (21) in combination with a diffuser (23) for polarizing the light supplied by the light source (11). The reflective polarizer (21) is integrated with the optical waveguide and constitutes the surface (25) of the optical waveguide (9) which is located opposite the exit surface (19). The diffuser (23) is present on a surface of the polarizer (21) remote from the optical waveguide (9).