To adjust the emission rate of radiation from an X-ray tube, the value of the current of the X-ray tube is modeled empirically by a second-order polynomial function depending on the heating current and a first-order polynomial function depending on the high voltage. A transfer function gives precision closer than 3% for the adjusting of an expected tube current. This function can also be used to take account of disparities of manufacture and of the aging of the tubes in use.