A planar-helical undulator for emitting 360° electrically variable photo radiation, including a first coil and a second coil disposed relative to an undulator axis, an axis of the first coil and an axis of the second coil and the undulator axis being parallel to each other, and the undulator axis forming a portion of a synchrotron beam axis. Further, each of the first and second coils includes a helical section and a planar section. The windings of each respective section are connected in series, so that the planar section generates, when energized, a first magnetic field, and so that the helical section generates, when energized, a second magnetic field. Each planar section is disposed around the corresponding helical section, and at least one of the helical section and the planar section of at least one of the coils includes variable windings changing symmetrically over a length of the respective section towards a middle of the respective section.