The illumination system has a plurality of light sources (R, G, B), a light-transmissive light-sensing plate (5) and surface-modification structures (21, 22) provided at a limited number of pre-determined locations on a surface of the light-sensing plate (5). The surface-modification structures divert a portion of the light traveling through the light-sensing plate. The diverted light is guided towards an edge surface (15, 16) of the light-sensing plate, where at least one light sensor (11, 12) senses the diverted light. A first light sensor (11) preferably receives diverted light substantially exclusively from the first surface-modification structures (21), whereas the second light sensor (12) receives diverted light substantially exclusively from the second surface-modification structures (22). Alternatively, the illumination system further comprises a light-mixing chamber having a light-emission window for illuminating a display device, the light-sensing plate being arranged in the light-mixing chamber between the light source and the light-emission window.