Advanced millimeter-wave imaging enhances security screening

Common security screening techniques at airports and other facilities have for many years consisted of metal detectors for personnel screening complemented by x-ray scanners for carried baggage. The primary weakness of these methods is that they cannot detect non-metallic items, such as explosives, concealed under the person's clothing. Since the exact nature of potential threats is unknown, we need to develop screening technologies that can detect essentially any anomalous object. Imaging systems that can penetrate clothing and reveal concealed items will allow a human operator or computerized automated threat recognition algorithm to differentiate concealed items from body features or innocuous items.

There have been a number of techniques developed for personnel screening, including active millimeter-wave (mm-wave), passive mm-wave, and backscatter x-ray imaging.^1–4 The passive mm-wave imaging systems operate in a manner similar to IR imaging systems that detect thermal emission or background reflection from the body. The active mm-wave imaging systems essentially measure the reflectivity of the body or concealed objects. Backscatter x-ray systems measure the x-ray energy scattered from the body or concealed objects. Each of these techniques has relative strengths and weaknesses. The passive mm-wave systems may have poor sensitivity and low contrast. Active mm-wave systems may suffer from poor image quality due to the nature of specular reflection. Backscatter x-ray systems use ionizing radiation, are often bulky, and scan at relatively low speeds.

We have focused on active mm-wave imaging because of its safety, high speed, high sensitivity, and 3D high-resolution capability. We have also highly optimized our scanning geometries to obtain images with excellent illumination and focusing properties, overcoming many of the limitations of initial active imaging techniques.

Figure 1. Millimeter-wave (40–60GHz) cylindrical images of a mannequin with concealed items. The left image shows a frontal view revealing a concealed glass vial on the torso. The right image reveals numerous small items concealed on the back of the mannequin, as well as a handgun barrel just off the left thigh.

We have developed a cylindrical imaging technique that simultaneously optimizes clothing penetration, system resolution, image quality, and inspection speed. Clothing is relatively transparent throughout the mm-wave frequency range, and these waves scatter from all concealed objects as well as the body. Obtaining a high-quality mm-wave image requires a wide aperture so that the diffraction-limited resolution is as fine as possible. This requirement has led us to use synthetic aperture techniques that allow for mathematical focusing without requiring optical focusing methods. This wide aperture has also allowed us to optimize the apparent image quality by illuminating each point on the person from a wide diversity of angles. The high sensitivity of active mm-wave imaging has allowed us to design high-speed sequentially switched linear arrays that can be used to quickly acquire image data from the entire body surface in as little as one to two seconds.

Our original cylindrical mm-wave imaging technique has been commercialized, and the imaging systems are rapidly gaining acceptance as an effective security tool to augment conventional systems. We recently developed a number of approaches that may improve the performance of existing systems. These include novel image reconstruction and display, polarimetric imaging, array switching, and high-frequency, high-bandwidth techniques. Figure 1 shows an example of a high-frequency 40–60GHz cylindrical imaging result.

To improve the resolution of the cylindrical mm-wave imaging technique, we are studying the use of higher-frequency and wider-bandwidth transceiver and array designs.^{5, 6} The resolution of the cylindrical imaging is limited laterally by diffraction and in depth by the spatial resolution obtained by sweeping a wide-frequency bandwidth. Using wide-beamwidth antennas allows for resolution on the order of one-half wavelength laterally, and wide-bandwidth operation can allow resolution comparable to the lateral resolution. Polarimetric imaging properties may also be used to enhance detection.

Some of these techniques will increase the cost and complexity of mm-wave security portal imaging systems. Reducing this cost may require the development of novel array designs. In particular, RF photonic methods may provide new solutions to the design and development of the sequentially switched linear mm-wave arrays that are the key element in mm-wave portal imaging systems. High-frequency, high-bandwidth designs are difficult to achieve with conventional mm-wave electronic devices. RF photonic devices may be a practical alternative and are currently being explored.

The cylindrical mm-wave imaging technique that we have developed allows for effective screening of personnel for essentially any concealed object, including weapons and explosives. The systems and techniques that are in current use can be improved through the use of higher-frequency, wider-bandwidth imaging arrays as well as those that can measure polarization properties. We are working to reduce the cost and complexity associated with implementation of these techniques.