In the present invention, a cathode for an x-ray tube is formed with a large area flat emitter. To reduce aberrations to a minimum the emission area in the flat emitter has a non-rectangular shape and focusing pads arranged around the emitter. In an exemplary embodiment, the flat emitter has a non-rectangular polygonal shape for an emission area on the emitter in order to increase the emission current from the emitter at standard voltage levels without the need to run the emitters at a higher temperature, add additional emitters to the cathode and/or to coat the emitters with a low work function material.