Even 100 years after Dr. Roentgen's detection, the basic X-ray tube design is nearly unchanged. A broad spectrum of poly-energetic X-rays is generated by deceleration of electrons in heavy metal, superimposed by the characteristic fluorescence lines of that material. Several physical effects, e.g. absorption and scattering, are energy-depending. A mono-energetic source, like a radio-nuclide, offers several advantages for numerous applications. An X-ray source combining mono-energetic radiation with the benefits of a switchable electrical device will be outlined. The major properties like spectral purity, stability and radiance will be presented followed by an outlook of potential application areas, especially in NDT.