7. Y.-L. Kuo and K.-L. Wong, Printed double-T monopole antenna for 2.4/5.2 GHz dual-band WLAN operations, IEEE Trans Antennas Propagat 51 (2003), 2187-2192. 8. K.-M. Chang, et al., A novel design of a CPW-FED square slot antenna with broadband circular polarization, Microwave Opt Technol Lett 48 (2006), 2456-2459. 9. B.Y. Toh and R. Cahill, Understanding and measuring circular polar- ization, education, IEEE Trans 46 (2003), 313-318. 10. Antenna Standards Committee, IEEE standard test procedures for antennas, ANSI/IEEE Std, 1979, pp. 1949-1979. © 2007 Wiley Periodicals, Inc. MILLIMETER WAVE INSPECTION OF CONCEALED OBJECTS Irina Jaeger, Lixiao Zhang, Johan Stiens, Hichem Sahli, and Roger Vounckx Department of Electronics Pleinlaan 2, Vrije Universiteit Brussel, 1050 Brussels, Belgium; Corresponding author: ijager@etro.vub.ac.be Received 4 April 2007 ABSTRACT: Imaging concealed objects with a millimeter-wave coher- ent beam is accompanied with speckle. Two tools were chosen to im- prove visibility of concealed objects for security or industrial inspection: a speckle contrast image (image processing tool) and a Hadamard dif- fuser (mechanical tool). We report more then 50% speckle reduction over the full W-band. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2733–2737, 2007; Published online in Wiley Inter- Science (www.interscience.wiley.com). DOI 10.1002/mop.22870 Key words: millimeter-wave imaging; microwave sensors, nondestruc- tive sensing, image processing 1. INTRODUCTION The millimeter-wave wavelength area belongs technologically to the boundary between the electronics and optics. Advantages of millimeter-wave radiation are ● radiation penetrates clothing and many packaging materials (but not suitable to penetrate metals); ● many substances have characteristically spectrums at this range; ● millimeter-wave radiation is nonionizing, and there are no known hazards to human health. Difficulties appear only if objects show a high concentration of water, because water absorbs millimeter waves especially strongly. These properties define a variety of the millimeter- wave imaging applications in the area of the industrial inspec- tions. This comprised both quality tests of products and super- vision of processes. Passive millimeter-wave imaging technology (due to the large temperature contrast between the radiation of the cold sky and the warm human body) is already capable of identifying metallic, plastic, explosives, and other hidden materials [1, 2]. Active mil- limeter-wave technology makes possible imaging of concealed objects indoor, where the temperature contrast between the walls and the body is typically very small. W-band illumination supports an imaging of concealed objects, providing both enough spatial resolution and good penetration. Imaging an object with a coherent beam is accompanied with speckle phenomenon—alternatively constructive and destructive interferences over the aperture and the field of view [3–5]. Speckle patterns are best described in statistical terms; there- fore, we will follow Goodman [3] and Trisnadi [5] and use the speckle contrast C = /I as a measure of speckle. It is defined as the ratio of the standard deviation  to the mean of the speckle intensity I, and its value is between 0 and 1. For fully developed speckle, contract patterns from a monochromatic light source C is 1. Contrast of nearly zero shows that illumination is approaching to speckle-free incoherent system. To study speckle contrast in W-band, we consider the setup shown in Figure 1. It is designed to meet aviation security appli- cations: the setup is based on a real-sized scanning system with angle diversity and object is being scanned on a special back- ground, which has a human-body-like reflection of around -9 dB. A backward wave oscillator (BWO) emits illumination over the whole W-band. The object under detection is automatically scanned with 5-mm step on an azimuthally plane. Telescopic two-lens system transforms the image pixel under detection from the object plane to the detector plane under different illumination angles. A single W-band planar detector with an open WR10 waveguide probe as receiving antenna is used to detect the signal reflected from the object. The derived image has a dimension of 60  50 pixels. 2. SPECKLE CONTRAST IMAGES OF CONCEALED OBJECTS When imaging concealed objects speckle arise from the surface and subsurface roughness. Three key experiments were systemat- ically prepared to demonstrate properties of speckles of the con- cealed objects: The first test was to determine if speckle produced by surfaces and subsurfaces are mixing. We chose a piece of rough metal surface of 10 cm  10 cm. To reveal wavelength features of an image, a maximum surface height of 3 mm was fabricated. We have calculated a speckle contrast image [Fig. 2(a)] and compared it to the speckle contrast image of the same rough surface, but covered with a white paper sheet. The grainy structure shown in Figure 2(a) is very typically for millimeter- wave images, where speckles are being produced not only by the surface of the object, but also by all the existing subsur- Figure 1 Imaging setup with angle diversity. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 11, November 2007 2733