Disclosed is a brightness control device for use with fiber optical lamps, comprising a diaphragm which is arranged unilaterally in the light beam between the lamp light source and the light entrance area of the fiber optical conductor. The diaphragm comprises a variable size diaphragm which is designed such that the cross-section of the light entrance area of the fiber optical conductor is asymmetrically and unilaterally reduced in order to maintain a constant lamp color temperature and emission characteristic.