Millimeter wave holographic imaging of concealed weapons

Experiments are described in which optical images of small metallic objects were reconstructed from millimeter wave phasigrams (phase-only holograms). Since the well-established term HOLOGRAM introduced first by Gabor implies a "total record," namely of amplitude and phase, the term phase-only hologram would be contradictory. The authors propose the term PHASIGRAM for a "phase record" based on the Greek work "phasis" for phase. Sufficient resolution is present in the images obtained to allow the identification of a toy metallic gun, even when covered with a layer of clothing or synthetic leather. The phase distribution of the 70-GHz millimeter wave field scattered by the object was determined by mechanically scanning a harmonic mixer over a 0.75-m diameter circular aperture in a spiral pattern. The resulting electrical phase signal was converted to a light intensity pattern which was recorded on photographic film. A He-Ne laser was used to illuminate a reduced transparency replica of the recorded phasigrams and reconstruct the images.