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W-band quasi-optical speckle contrast measurements with a Hadamard diffuser”, Society of Photo-optical Instruments (SPIE), Vol. 6341, pp-U19-U24 , 2006 Host Publication: PROCEEDINGS OF THE SOCIETY OF PHOTO-OPTICAL INSTRUMENTATION ENGINEERS (SPIE) Authors: S. Islam, I. Jäger, G. Koers and R. Vounckx Publication Date: Sep. 2006 ISBN: 0-8194-6426-0
Abstract: 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.
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