Millimeter wave imaging is finding rapid adoption in security applications such as the detection of objects concealed under clothing. A passive imaging system can be realized as a stand-off type sensor that can operate in open spaces, both indoors and outdoors. In this paper, we address real-time outdoor concealed-object detection and segmentation with a radiometric imaging system operating in the W-band. The imaging system is equipped with a dielectric lens and a receiver array operating at around 94 GHz. Images are analyzed by multilevel segmentation to identify a concealed object. Each level of segmentation comprises vector quantization, expectation-maximization, and Bayesian decision making to cluster pixels on the basis of a Gaussian mixture model. In addition, we describe a faster process that adopts only vector quantization for the first level segmentation. Experiments confirm that the proposed methods provide fast and reliable detection and segmentation for a moving human subject carrying a concealed gun.
The image space of the reconstructed image from the hologram displayed on a digital micromirror device (DMD) is defined by the diffraction pattern induced by the 2D pixel pattern of the DMD, which works as a 2D blazed grating. Within this space, a reconstructed image of 100 mm × 20 mm is spatially multiplexed by a 2 × 5 DMD array that is aligned on a board, without using any extra optics. Each DMD chip reconstructs an image piece of the size 20 mm (width) × 10 mm (height). The reconstructed image looks somewhat noisy but regenerates the original object image faithfully.
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