Stand-off detection for concealed weapons is one of the applications for passive submillimetre-wave imaging. The operating frequency (neglecting technology limitations) is often a compromise between the diffraction-limited angular resolution for a fixed maximum aperture diameter, and the extinction of the signal in obscurant layers: At high frequencies towards the 1 THz mark, excellent angular resolution is readily achievable with modest aperture diameters, while scattering and attenuation by clothing is high which creates potentially more clutter rather than improving detection capability. At lower frequencies towards 100 GHz, attenuation and scattering by clothing is much less pronounced, albeit at significantly reduced spatial definition thanks to increased diffraction. In order to avoid the above-mentioned compromise, we have constructed a three-band passive imaging system operating at effective centre frequencies of 250 GHz, 450 GHz and 720 GHz. Aspects of the system will be presented.