Goldsmith et al. (1993) aim to develop a focal plane array to operate at 3mm wave- length. Phased arrays have been suggested but developments have been limited by cost and complexity. Focal plane arrays have multiple feeds, a single focussing element, and multiple beams. A mechanically scanned single element produces a single beam, the scanner is moveable in all directions. It requires good illumination of the focussing element by the feed for a focal plane array. A combination of Gaussian beam optics and ray optics is used to simulate the array properties. Development of millimetre wavelength transistor amplifiers can be applied to imaging arrays. The advantages of frequencies between 30 and 300 GHz are low absorption by most dry dielectric ma- terials, moderately high spatial resolution and compact optics. A scanned beam of millimetre wavelengths can suffer from ‘glints’ from edges of clothing etc. Glints are spurious bright reflections, usually from metallic objects.
Watabe et al. (2003) uses a feed forward neural network in pattern recognition for a real-time imaging system. The network is trained to recognise metallic letters. The operating frequency was 60 GHz using an Yagi-Uda array. To reduce the size of the images to input into the neural network, adjacent pixels were averaged with a 2 by 2 averaging mask. This reduced the number of pixels to a quarter of the original number. Federici et al. (2005) present a review of terahertz imaging systems used to de- tect explosives and drugs. In the THz band, explosives and drugs have characteristic transmission or reflection spectra which can be used to detect these substances when concealed on a person. THz spectroscopy can be used to detect explosives and drugs through concealing materials such as clothing, plastics and cardboard because these materials are transparent to THz. By comparing measured spectra with known cali- bration spectra it is possible to detect and distinguish explosives and drugs from other benign objects, but such a system would have to store a large number of reference spec- tra to be practical. Federici et al. (2005) state that, in general, non-polar, non-metallic solids such as plastics and ceramics are at least partially transparent and reflective in
such as water, are highly absorbing. This is because absorption in the THz range of the electromagnetic spectrum is generally due to rotational motions of dipoles within a material. This is also true for the millimetre wave range of the electromagnetic spectrum.
One disadvantage of the spectral analysis technique is that features of the THz spectra can be sensitive to the way a sample is prepared, e.g. pellitized form. However, there are many spectral features of explosives which are reproducible and not sensitive to sample preparation so could be used as a method of detecting explosives.
A comparison of focal plane arrays and an interferometric technique was presented by Federici et al. (2005) and it was found that the focal plane array would take 30 times longer than the interferometric approach to acquire an image. Although the interferometric technique required more imaging processing (correlating) therefore it would be difficult to produce an image in real-time.
Federici et al. (2005) conclude that for close range scanning CW or pulsed modes of operation should be competitive, whereas real-time imaging needs development. THz spectra detection has some limitations on the type of materials it can detect, for example some explosives do not have a THz spectra below 3 THz. Stand-off detection is a big challenge because as the distance increases the attenuation by the atmosphere, dust and smoke increases; to overcome this, higher power sources need to be developed. Doyle & McNaboe (2006) describe a mechanically scanned millimetre wave imaging camera capable of concealed object detection at stand-off distances up to 50m. Range information can be obtained by using FMCW (frequency modulated continuous wave) radar. The system only uses reflective optics, to minimise losses in the optical path. This system can be used in either portal or standoff mode. For use in portal mode it includes a thermally generated illumination scheme to improve image contrast. Metal objects were detected on people at distances up to 20m, with a scan time of 2 to 4 seconds. Combining the range function of FMCW radar and a video image of a scene in real-time would be the ideal user interface for a stand-off weapons detector.
scanned imaging of a target in azimuth and elevation using two-axis Fourier Transforms. A prototype system fast scans a small spot across a target. High resolution images were obtained using a technique similar to synthetic aperture radar.
HEMT (high electron mobility transistor) amplifiers have been developed for the 220 GHz band by Essen et al. (2007). The radar range resolution is determined by the bandwidth of the radar, large bandwidth is easier to achieve at higher frequencies. The radar developed by Essen et al. (2007) has a bandwidth of 8GHz. Imaging of a person with and without a gun was carried out, with a range resolution of 2cm.
Derham et al. (2007) present a prototype imaging system operating in the 60 GHz band using the frequency encoding technique. A single channel coherent receiver is used, the signal contains angular information about the scene encoded in frequency. Processing with a FFT constructs a one-dimensional image, each FFT point repre- sents one of the beams of the antenna. The system was tested with a background of microwave absorber as shown in figure 3.3.
Figure 3.3: The top figure shows the optical image of two mannequins, the bottom figure shows the MMW image of the two mannequins to demonstrate the imaging performance of the system designed by Derham et al. (2007).
The outline of the mannequin is indistinct for both direct observation and through a thin plywood panel as shown in figure 3.4. Specular reflections from the flat panel caused signification image distortion.
Figure 3.4: The figure shows the MMW image of a mannequin through a 3 mm plywood panel, with the detector on axis and then 30 degrees off axis (Derham et al., 2007).
Volkov et al. (2008) show that in indoor use the image contrast in a passive system is not enough for optimum imager operation (Volkov et al., 2008), artificial illumination mimics the sky illumination. An edge detection system is presented, which separates the return from concealed objects from the background such as the body. The system is fixed at a frequency of 94 GHz and uses a cassegrain mirror. The system provides a spatial resolution of 7 mm at a distance of 1.2 metres.
Two technologies are being tested in US and UK airports, x-ray back scatter imag- ing; using very low energy x-rays and millimeter-wave imaging (as just described). There are concerns over privacy due to constructing images of people. The manufac- turers claim that these scanners pose no risk to human health according to Brown (2008).
Cooper et al. (2008) present a technique for active imaging using a 600 GHz radar. They use an image processing algorithm which will determine if the pixel belongs to the front or back surfaces of the target and use this to determine if there is a potential threat present. The ranges to the nearest and furthest object in the radar beam produce front and back peak pairs. Some peaks require manual re-selection after the image is constructed by the image processing algorithm as they are missed in the automatic process (Cooper et al., 2008). The image collection time was around 6 minutes.