An oxide sintered body is composed mainly of indium and containing tungsten, has a resistivity of no more than 1 k\u03a9cm. A second power amplifier amplifies a second input radio frequency signal in a second frequency band in response to a second bias control signal. The image is truncated by a variable aperture at or near the aperture plane conjugate to the image plane, to block progressively increasing portions of an incident coherent radiation beam used to form the line image.