Illumination strategies to achieve effective indoor millimeter wave imaging for personnel screening applications

Abstract

The ability of millimetre-waves (mm-wave) to penetrate obscurants -be they clothing, fog etc. -enables unique imaging applications in areas such as security screening of personnel and landing aids for aircraft. When used in an outdoor application, the natural thermal contrast provided by cold sky reflections off of objects allow for direct imaging of a scene. Imaging at mm-wave frequencies in an indoor situation requires that a thermal contrast be generated in order to illuminate and detect objects of interest. In the case of a portal screening application the illumination needs to be provided over the imaged area in a uniform, omni-directional manner and at a sufficient level of contrast to achieve the desired signal to noise ratio at the sensor. The primary options are to generate this contrast by using active noise sources or to develop a passive thermally induced source of mm-wave energy. This paper describes the approaches taken to developing and implementing an indoor imaging configuration for a mmwave camera that is to be used in people screening applications. The camera uses a patented mechanical scanning method to directly generate a raster frame image of portal dimensions. Imaging has been conducted at a range of frequencies with the main focus being on 94GHz operation. Experiences with both active and passive illumination schemes are described with conclusions on the merits or otherwise of each. The results of imaging trials demonstrate the potential for using mm-wave imaging in an indoor situation and example images illustrate the capability of the camera and the illumination methods when used for personnel screening.

Millimeter-Wave and Terahertz Imaging in Security Applications

“…(Black-is-"hot"). Portal images c using spatially incoherent illumination at 35 GHz from a noise source inside a mode scrambling cavity [13] and d at 94 GHz from thermally warm panels illustrating concealed threats as white [14]. (White-is-"hot") Polarization has not been shown to be a powerful discriminator in passive MMW imaging, as the degree of linear polarization is generally below 20 % and that of circular polarization below 1 % [11].…”

Human Presence Detection Using Millimeter-Wave Radiometry

“…However, the millimeter-wave radiometric temperature of the sky is approximately 20 K due to the cosmic microwave background [7], and in contrast, nearly any material will have a larger brightness temperature. Other millimeter-wave imagers use active technology for contraband detection, transmitting a signal and detecting the reflected millimeter-wave radiation from the person and any objects under the person's clothing [8,9]. However, in a security environment it is beneficial to have a passive sensor so that no information is being transmitted that could possibly be intercepted, indicating to an intruder where the sensor might be.…”

Development of passive millimeter wave imaging for concealed weapon detection indoors

“…Reconfigurable antennas have been extensively studied due to their advantages of a high degree of polarization control and frequency reusability for enlarging the capability of wireless communications systems [1][2][3][4]. In previous researches, the reconfiguration of polarization diversity is discussed for linear polarization (LP) to LP [5], LP to circular polarization (CP) [6], and CP to CP, that is, right-handed CP (RHCP) to left-handed CP (LHCP) and vice versa [7,8].…”