Abstract: An X-ray tube has a control grid located to cover a filament which is insulatingly mounted on a first focusing electrode. The control grid is electrically connected to the first focusing electrode. A second focusing electrode is so mounted on the first focusing electrode through an insulator that it covers the control grid. The second focusing electrode is made at the same potential level with that of the filament.

Abstract: An X-ray tube comprises an anode including a target giving off X-rays when electrons emitted from the filament of the cathode impinge upon the target and being heated at this time, and a supporting structure for supporting the target and being subject to transfer of the heat produced on the target, the anode being plated with black chromium the black chromium plating is not applied onto the electron striking surface of the target. The thickness of the chromium plating is 0.05 to 0.2 .mu.m, preferably 0.05 to 0.1 .mu.m. The roughness of the plated surface is 0.5 to 20 .mu.m, preferably 5 to 20 .mu.m. In the rotating anode X-ray tube, for example, the black chromium plating, in place of the conventional blacking treatment by copper sulfide, is applied onto the outer surface of the rotor. In this case, the radiation efficiency for the heat rays with the wavelength of about 4 .mu.m is obtained approximately three times as large as that of the conventional one.