Active millimeter-wave imaging for concealed weapons detection

Actively illuminated mm-wave imaging systems for concealed weapons detection offer the possibility of much higher sensitivity than with passive systems. We discuss several issues specific to mm-wave imaging systems that are actively illuminated and describe our implementation of such a system. We show that the illumination geometry is critical to the appearance and detectability of targets, and present measurements of the bi-directional reflectance distribution of a small handgun, as a prototypical target. We demonstrate suppression of standing waves using frequency modulated (FMCW) illumination and directly compare images taken with CW, FMCW, and pulsed illumination. Our implementation of an active imaging system is based on a 120-element focal-plane array (FPA) of uncooled antenna-coupled microbolometers. We describe the fabrication process and yield for these FPA's, as well as tests that show the performance of individual pixels on the 120-element array to be equivalent to that of isolated devices. These results demonstrate that wiring for readout of an antenna-coupled FPA can be made "transparent" to the antennas. Therefore, a mm-wave FPA that is read out through wiring to the perimeter of the array constitutes a simple and feasible architecture, in contrast to the infrared, where space limitations require vertical (3D) integration of readout and FPA. The electrical readout system for the FPA is also described. Finally, we present a result obtained with the full FPA system, showing that 3-way illumination is much more effective for detection of a complex target than a single directional illuminator.