The present invention relates to an illumination system having an array of differently coloured LEDs, a plurality of collimators, wherein each collimator holds at least one LED, a light mixing element, arranged to mix light generated by the LEDs into a mixed light that is emitted from the illumination system, and a control system for controlling the colour balance of the mixed light. The control system comprises a detector, which is placed among said collimators and encircled by a light shield. The detector detects a fraction of the mixed light that is reflected at an exit window of the mixing element. In this structure the detected light twice passes the mixing element, which provides for a proper mixing of the different colours as well as a proper homogenisation of the light.