If an incoming free electron gets close to the nucleus of a target atom, the strong electric field of the nucleus will attract the electron, thus changing direction and speed of the electron. The electron looses energy which will be emitted as an X-ray photon. The energy of this photon will depend on the degree of interaction between nucleus and electron, i.e. the passing distance. Several subsequent interactions between one and the same electron and different nuclei are possible. X-rays originating from this process are called bremsstrahlung. Bemsstrahlung is a German word directly describing the process: "Strahlung" means "radiation", and "Bremse" means "brake".

The process can create photons of practically all energy values between zero and the maximum determined by the total kinetic energy of the incoming electron. The chances for the generation of a photon with a certain energy by this process decreases with increasing energy and reaches practically zero for the very unlikely event that an incoming electron looses all its energy in one single interaction. Consequently, the resulting radiation contains photons of practically all energy values between zero and the maximum. The distribution of the relative number of photons with a certain energy, as a function of that energy, will decrease with increasing energy and will reach zero at the maximum energy. This is equal to the energy the electron picked up during the acceleration by the electric field between cathode and anode. This energy is conveniently measured in electron volts: one electron volt (eV) is the energy acquired by an electron traveling through a potential difference of one volt. Therefore, accelerating electrons in an X-ray tube with a voltage of x kV will yield electrons with an energy of x keV, and this will also be the maximum energy an X-ray photon emitted by this tube can have.