The illumination system has a plurality of light emitters (R, G, B) and a light-collimator (1) for collimating light emitted by the light emitters. Light propagation in the light-collimator is based on total internal reflection (TIR) towards a light-exit window (4) of the light-collimator. At least one light sensor (8) for optical feedback is placed outside the light-collimator and is arranged to receive light emitted by the light emitters exclusively through reflection at the light-exit window of the light-collimator. Preferably, the light sensor is placed substantially coplanar with the light emitters. Preferably, a side wall (35) of the light-collimator is provided with a protruding portion (9) for guiding the light reflected at the light-exit window of the light-collimator towards the light sensor. Preferably, the illumination system is provided with a reflector (12). Preferably, the illumination system comprises a holographic diffuser (17). Accurate sensing of the color mixing of light emitted by the illumination system is obtained.