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We report on the development of an active stand-off imaging system operating in the 80 GHz - 110 GHz frequency range. 3D real-time imaging is enabled by a combination of a mechanically scanned one-dimensional conventional imaging projection with a rotating metallic reflector and a two-dimensional synthetic imaging reconstruction with a linear array of transmitter (Tx) and receive r (Rx) elements. The system is conceived, in order to allow a resolution better than 1cm both in lateral, as well as in range directions by using a multi-view imaging geomet ry with an aperture larger than 2 m x 2 m. The operation distance is 8.5 - 9 m. The 2D synthetically reconstructed imaging planes are derived from the correlation of 20 sources and 24 coherent detectors. Range information is obtained by operating in a frequency modulated continuous wave (FMCW) mode. Real-time imaging is enabled by implementing the synthetic image reconstruction algorithms on a general purpose graphics processing unit (GPGPU) system. A multi-view imaging geometry is implemented, in order to enhance the imaging resolution and to reduce the influence of specular reflections. Keywords: THz, millimeter-wave, imaging, stand-off, FMCW, deramping, synthetic reconstruction. *matthias.kahl@uni-siegen.de

An active system for stand-off imaging operating in a frequency range from 234 GHz to 306 GHz is presented. Imaging is achieved by combining a line array consisting of 8 emitters and 16 detectors with a scanning cylindrical mirror system. A stand-off distance of 7-8 m is achieved using a system of mirrors with effective aperture of 0.5 x 0.5 meter. Information about range and reflectivity of the object are obtained using an active FMCW (frequency modulated continuous wave) radar operation principle. Data acquisition time for one line is as short as 1 ms. Synthetic image reconstruction is achieved in real-time by an embedded GPU (Graphical Processing Unit).

We report on the realization of two active fully electronic THz cameras operating at different frequencies (645 GHz and 300 GHz) and room temperature. Active illumination together with the frequency modulation continuous wave approach allows us to implement unique features, such as phase-sensitive detection, working-distance selection and the suppression of spurious reflections. With both systems we are able to acquire images with more than 50000 pixels (phase and amplitude) in 9 seconds. The dynamic range exceeds 35 dB and we achieve a subwavelength depth resolution due to the measurement of the phase. The typical object distance is about 50-100 cm and the image size is on the order of hundreds of cm2. With frequency modulation of the source we are furthermore able to detect the position form objects up to an accuracy of a few mm.