Disclosed is for controlling a light source by pulse width modulation of a supply voltage. The supply voltage, or a parameter dependent thereon, is measured and the pulse width is controlled as function of the measured value. The supply voltage or parameter is measured at least twice during the pulse and may be cyclically measured. The pulse width of the current or a subsequent pulse is matched to the recorded value of the supply voltage or the parameter. A total value is generated from all of the measured values and compared with a given value and the pulse width of subsequent pulses are matched as function of the difference between the total value and the given value.