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The development of coherent light sources has made possible new optical radar systems in which target information should be determined with a far greater accuracy than would be possible with a conventional microwave radar system. However, the same factors which make the improved resolution possible impose difficult requirements on the optical receiver. Presently available image detectors have very limited frequency response and are not suitable for use in a Doppler or ranging system. This paper describes the design and experimental evaluation of an image detector with an instantaneous bandwidth of 4 Gc/s. This photodetector is capable of providing simultaneous target velocity and target position information when the Doppler shifted frequency is in the range of 0 to 4 Gc/s. The device consists of an image dissector electron gun and a traveling-wave output coupler. The scanning of an optical field of view for microwave-modulated light signals was demonstrated experimentally by a direct modulation-demodulation experiment with a modulation frequency of 10.7 Gc/s.