A light source with - at least two semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212), the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) emitting light of different colour, the light source comprising: - a light guide into which the light emitted by the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) is injected, at least partially, and wherein the light guide is configured such that the light injected by the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) exits from the light guide laterally; - an electronic control unit adapted to adjust the brightness of the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212); and - a light sensor arranged so as to receive the light injected by the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) into the light guide and laterally exiting therefrom, wherein a spacing is kept along the longitudinal extension of the light guide between the light sensor and the end face of the light guide, and wherein the electronic control unit with the light sensor is adapted to: - accumulate the sensor signals over an integration time interval; and - compare the accumulated signals of the light sensor with a target value or target range and to modify the brightness of the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) in response to the difference between the sensor signals and the target value or target range, and to alter the integration time interval in response to the difference between the sensor signal and the target value or target range or in response to a change in the target value. - at least two semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212), the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) emitting light of different colour, the light source comprising: - a light guide into which the light emitted by the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) is injected, at least partially, and wherein the light guide is configured such that the light injected by the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) exits from the light guide laterally; - an electronic control unit adapted to adjust the brightness of the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212); and - a light sensor arranged so as to receive the light injected by the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) into the light guide and laterally exiting therefrom, wherein a spacing is kept along the longitudinal extension of the light guide between the light sensor and the end face of the light guide, and wherein the electronic control unit with the light sensor is adapted to: - accumulate the sensor signals over an integration time interval; and - compare the accumulated signals of the light sensor with a target value or target range and to modify the brightness of the semiconductor light-emitting elements (21, 22, 23, 24, 210, 211, 212) in response to the difference between the sensor signals and the target value or target range, and to alter the integration time interval in response to the difference between the sensor signal and the target value or target range or in response to a change in the target value.