The present invention relates to a system (100) for controlling light output of a lighting system. The system (100) comprises a light mixing circuit (116) comprising a plurality of light sources configured to provide a mixed light output (102) and mounted on a heat-sink (202) together with a temperature sensing means and a controller (108) receiving a set-point (110) from a calibration matrix (104) and generating a driving signal (120, 122) for the light mixing circuit (116). The controller (108) comprises a rescale unit (118) configured to measure power of the driving signal (120, 122) and to rescale the driving signal (120, 122) when the power exceeds a predetermined power threshold, and the controller is configured to receive the heat-sink temperature signal (206) and to calculate a junction temperature from the heat-sink temperature signal, and the controller (108) generates the driving signal (120, 122) as a function of the junction temperature.