This paper presents the design, construction and testing of grounded Frequency Selective Surface (FSS) array as
millimeter wave beam splitter. The phase dependence on slot length of grounded FSS demonstrates that the reflection
phase of coherent mm-wave can be altered by using FSS array with different slot lengths. A beam splitter was designed
with slot FSS array where the slot length is the main design parameter used to optimize the phase properties of the array.
We simulated the FSSs with commercial CST Microwave studio software, fabricated them with etching technique and
characterized with a free space MVNA and BWO with motorized detector setup.
Active millimeter wave imaging technology is emerging, which has the potential to yield much more information when
one has control over the illumination parameters. Image processing of this kind of images is almost inexistent in
literature. In this paper, we propose multidimensional illumination techniques to improve the mm-wave image quality.
Multi-angle, multi-frequency, and cross-polarization illuminations were implemented to obtain multidimensional images.
Principle Component Analysis (PCA) and clustering analysis were applied to process the results.
Free-space active W-band millimeter-wave imaging (75-110 GHz) makes possible imaging of phenomena,
inaccessible to visible and infrared light. W-band supports the imaging of concealed objects, providing both
enough spatial resolution and good penetration. An advantage of mm-wave radiation over X-ray is that it is
non-ionizing, and there are no known hazards or risks to human health. When imaging an object with an mmwave
coherent beam, this is accompanied with speckle phenomenon. Because mm-wave wavelength is
closer to the surface roughness and to the object dimension as by optical imaging, spatial distribution of
speckle gives us more information than the image itself.
We will use a speckle contrast as a measure of the speckle. Speckle contrast contains useful information
when it differs from unity, and has been utilized here to reveal surface roughness of concealed objects. The
speckle contrast starts to be reduced from unity when an incoherent part compensates coherent light. A
sequence of mm-wave images was acquired with a fixed angle interval. The speckle contrast of each pixel in
the image was calculated and a new image was formed: a spatial speckle contrast image. It revealed areas,
covered with interference. Comparing the two images together makes all features of the hidden object visible.
We also present results, which illustrate mechanical speckle contrast reduction in full W-band by means of
phase diversity Hadamard solution. Hadamard principle has been proven by experimental conversion of the
coherent sum of the electrical millimeter wave amplitudes into an incoherent sum of intensities. The measured
data give results on speckle contrast reduction that match accurately the theoretical statistical estimations.
Industrial and medical imaging of concealed objects could benefit both from speckle contrast images and Hadamard speckle reduction.
Millimeter waves are able to penetrate materials that are usually opaque to both visible and infrared radiation. We used this advantage to design a free-space active millimeter-wave imaging set up for security applications. Because all existing mm-wave sources are coherent - speckle is one of the ultimate limiting factors of the imaging techniques. This problem is of special importance for mm-wave imaging, because surface roughness is closer to the object dimension as in optical imaging. The reduction of speckle is highly desirable and we propose here a Hadamard matrix solution, which is one of the most efficient ways to reduce speckle noise. By illuminating the object with a series of orthogonal phase patterns corresponding to permutations of Hadamard matrices, one can convert the sum of electrical fields (coherent) in the image pixel into a sum of intensities (non-coherent). We report a 50% speckle reduction. The effect of speckle reduction was measured using both a vector network analyzer and a W-band free-space scalar calibrated measurement setup. We processed 2 different Hadamard diffusers and present here measurement data discussing speckle contrast as a function of frequency.
In this paper a simplified "1.5D" modeling approach is presented which can be used to characterize and optimize an
entire active millimeter wave imaging system for concealed weapon detection. The method uses Huygens' Principle to
compute one field component on selected planes of the imaging set-up. The accuracy of the method is evaluated by
comparing it to a rigorous 2D method of moments approach. The model includes the effects of lenses, diffusers, mirrors,
object and any other component present in the system. The approach allows fast determination of the influence of each
of the system components on the image projected onto the sensor, including effects such e.g. speckle. Also, the
effectivity of different speckle reduction techniques, e.g. using a Hadamard diffuser or a multifrequency approach are
evaluated in this paper.
THz technology makes possible imaging of phenomena, inaccessible to both visible and infrared radiation, but the
imaging is still in its early stages of development. This paper draws attention to the aspects of speckle reduction to
improve the image quality. Because all existing THz sources are coherent - speckle is an ultimate limiting factor of the
free-space imaging techniques. Speckle arises when coherent light scattered from a rough surface is detected by an
intensity detector with a finite aperture, hiding the image information. This problem is of special importance for THz
imaging, because surface roughness is closer to the object dimension as in optical imaging. The reduction of speckle is
highly desirable and we propose here a Hadamard matrix solution. Hadamard diffuser for mm-wave frequency range
have been designed, built and tested. We report 50% speckle reduction measurements using a free-space vector network
analyzer over the full W-band (75-110 GHz). The advantage of the mm-wave Hadamard technology over optical: the
diffuser doesn't have to be moved (vibrated) any more to accomplish the technology of speckle reduction. Temporal
optical effect is substituted here by spatial quasi-optical: Hadamard coding in each scan pixel. Second method delivers
realistic system parameters for the speckle reduction with polychromatic light for aviation security.