The present invention relates to an apparatus for generating X-rays. It is described to produce (210) with a power supply (40) at least two voltages between at least one cathode (20) and an anode (30), wherein the at least two voltages comprises a first voltage and a second voltage. The at least one cathode is positioned relative to the anode. Electrons are emitted (220) from the at least one cathode. Electrons emitted from the at least one cathode are interacted (230) with the anode with energies corresponding to the at least two voltages. X-rays are generated (230) from the anode, wherein the electrons interact with the anode to generate the X-rays. First X-rays are generated when the power supply produces the first voltage and second X-rays are generated when the power supply produces the second voltage. The power supply is controlled (250), such that a ratio between the first X-rays and the second X-rays is controllable.