By placing interfaces (foils) of very precise thickness and foil separation, coherence effects will modify the transition radiation's spectral and angular characteristics. This allows a much higher number of photons to be obtained in a smaller angular "volume". Applications of this x-ray source are limited by the fact that the radiation is emitted in a cone, with a minimum intensity at the center. X-ray focusing devices (crystals/mirrors) are not easy to build for such radiation patterns.