Concealed weapons detection system using uncooled, pulsed, imaging arrays of millimeter-wave bolometers

We describe a system for detection of concealed weapons based on imaging the millimeter-wave reflectance contrast between the weapon and the human body. An architecture based upon pulsed illumination and gated integration has a large advantage in sensitivity over chopped CW illumination and synchronous detection. A simple sensitivity estimate based on an assumed range of 8 m, a 30 Hz frame rate, a primary collection area of 30 cm diameter, and an overall optical efficiency of 50 %, yields a noise-equivalent reflectance difference of 0.66 % in a single frame. The uncooled niobium microbolometers provide a (measured) noise-equivalent power of 100 pW/Hz(1/2) and a time constant of 175 ns, well matched to the source pulse width and the system sensitivity requirements. Optical coupling is provided by resonant slot-ring antennas, distributed in a focal plane array covering a single 75 mm diameter wafer. The optical and electrical systems used for coupling signals into and out of the focal plane array are also described.