Characterisation and calibration of a large aperture (1.6 m) ka-band indoor passive millimetre wave security screening imager

A large 1.6 m aperture mechanically scanning quasi-optical millimetre wave imaging system designed for active use [1] has been investigated for its passive millimetre wave (PMMW) imaging capabilities. This raises new challenges for this imager, as the person / background contrast in indoor security scenarios at these frequencies is in the region of 6-7 K. The system is sensitive over 24 – 28 GHz and has a radiometric sensitivity measured at 6.5 K in a single frame, at a frame rate of 8.8 Hz. It was found that the focal plane receiver array radiated an excess noise ratio (ENR) of typically 9 dB. This radiation re-entered the focal plane array receivers after reflection from the imager optical components typically with a standard deviation of 17 K over the mechanical scan associated with a single image frame. Reflected from subjects in the field of view, it was typically 400 K from a 100 % reflective surface, and 40 K to 100 K from a human subject. This radiation, termed self-emission in this paper, is a feature of many PMMW imaging systems that use highgain electronic amplifiers in their receiving systems. The effectiveness in the use of a calibration mechanism to subtract self-emission reflected from the optical components and a large area quarter wave plate to minimising the amount of self-emission reflected back from subjects is demonstrated. The present system would benefit greatly from replacement amplifiers of larger bandwidth, lower self-emission and lower noise figure. It was also found that the millimetre wave emission from fluorescent room lighting entered the optics of the imager and modulated the received signals at 100 Hz. The mean level of emission was measured at 10 K with a 10 K modulation at 100 Hz for per mode for a 100 % reflecting surface in the room. The best solution to the problem of emission from fluorescent lights was to turn them off and use incandescent lighting