Automatic exposure control in tomography requires control of dose output whereas in survey technique dose is controlled. Automatic exposure not only facilitates work load but also improves image quality by balancing regions of differing density. High control-frequency and -sensibility are essential requirements; otherwise unsatisfactory findings i.e. erraneous images may result in particular when the time for a tomographic cycle is short. These high requirements exceed the performance margin of conventional automatic exposure control systems in which dose control is achieved by control of tube heat. A combined tube-current- and voltage-control with regard to primary voltage phase as used in units designed lately serves all routine use needs and delivers high quality tomograms.