Method for controlling an illumination system comprising a plurality of coloured light sources, there being at least one or more light sources of a first colour and one or more light sources of a second colour, the first colour being different from the second colour, the illumination system being for emitting illumination light, and having an optical sensor coupled to a sample and hold circuit, the method comprising determining first drive settings for each of the plurality of coloured light sources so as to provide illumination light with a pre-determined colour point, the first drive settings generating an ON time and an OFF time of the plurality of coloured light sources, the first drive settings comprising current control and pulse width modulation control with pulse width modulation pulses; for each of the light sources of the first colour and for each of the light sources of the second colour performing a measuring step, the measuring step comprising - changing the first drive settings so that the ON time of one or more first light sources of a selected first one of the first and second colours does not coincide with the ON time of the light sources of other colours for at least a first period of time, and - during that first period of time, measuring a first peak luminance value of the one or more first light sources wherein the step of measuring comprises the step of sampling and holding the peak luminance value, and saving said first peak luminance value in a memory; - changing the first drive settings so that the ON time of one or more second light sources of a selected second one of the first and second colours does not coincide with the ON time of the light sources of other colours for at least a second period of time, and - during that second period of time, measuring a second peak luminance value of the second light sources, wherein the step of measuring comprises the step of sampling and holding the peak luminance value, and saving said second peak luminance value in a memory; and for each of the light sources of the first colour and the second colour performing a calculation step, based on the measured first and second peak luminance values for light sources of that colour, the calculation step comprising recalculating the drive settings as second drive settings so as to maintain said pre-determined colour point, furthermore comprising directly or indirectly measuring temperature of the coloured light sources, wherein if pulse width modulation pulses of the pulse width modulation control are too short to be sampled, being shorter than the addition of a response time of the optical sensor and a sample time, then switching to a temperature control algorithm based on lookup tables and last luminance measurements, to maintain said pre-determined colour point. - changing the first drive settings so that the ON time of one or more first light sources of a selected first one of the first and second colours does not coincide with the ON time of the light sources of other colours for at least a first period of time, and - during that first period of time, measuring a first peak luminance value of the one or more first light sources wherein the step of measuring comprises the step of sampling and holding the peak luminance value, and saving said first peak luminance value in a memory; - changing the first drive settings so that the ON time of one or more second light sources of a selected second one of the first and second colours does not coincide with the ON time of the light sources of other colours for at least a second period of time, and - during that second period of time, measuring a second peak luminance value of the second light sources, wherein the step of measuring comprises the step of sampling and holding the peak luminance value, and saving said second peak luminance value in a memory; and