A light source is provided that includes at least two semiconductor light-emitting elements that emit light of different color, a light guide, an electronic control unit, and a light sensor. The light emitted by the elements is injected, at least partially, into the light guide and exits laterally from the light guide. The brightness of the elements can be adjusted by the electronic control unit. The light sensor is arranged to receive the light injected by the elements and laterally exiting from the light guide. The electronic control unit accumulates sensor signals from the light sensor over an integration time interval and compares the accumulated signals with a target value or range to determine a difference, changes a brightness of the elements in response to the difference, and changes the integration time interval in response to the difference or to a change in the target value.