W-band speckle contrast images for inspection of concealed objects

Abstract

Free-space active W-band millimeter-wave imaging (75-110 GHz) makes possible imaging of phenomena, inaccessible to visible and infrared light. W-band supports the imaging of concealed objects, providing both enough spatial resolution and good penetration. An advantage of mm-wave radiation over X-ray is that it is non-ionizing, and there are no known hazards or risks to human health. When imaging an object with an mmwave coherent beam, this is accompanied with speckle phenomenon. Because mm-wave wavelength is closer to the surface roughness and to the object dimension as by optical imaging, spatial distribution of speckle gives us more information than the image itself. We will use a speckle contrast as a measure of the speckle. Speckle contrast contains useful information when it differs from unity, and has been utilized here to reveal surface roughness of concealed objects. The speckle contrast starts to be reduced from unity when an incoherent part compensates coherent light. A sequence of mm-wave images was acquired with a fixed angle interval. The speckle contrast of each pixel in the image was calculated and a new image was formed: a spatial speckle contrast image. It revealed areas, covered with interference. Comparing the two images together makes all features of the hidden object visible. We also present results, which illustrate mechanical speckle contrast reduction in full W-band by means of phase diversity Hadamard solution. Hadamard principle has been proven by experimental conversion of the coherent sum of the electrical millimeter wave amplitudes into an incoherent sum of intensities. The measured data give results on speckle contrast reduction that match accurately the theoretical statistical estimations. Industrial and medical imaging of concealed objects could benefit both from speckle contrast images and Hadamard speckle reduction.