Abstract-In passive millimeter-wave imaging systems used indoors, the radiometric temperature contrast is barely enough for coarse object detection, being usually insufficient for recognition due to the absence of cold sky. The image contrast results from a combination of emissivity and reflectivity which are dependent on the dielectric constant of objects, the angle of incidence, and the polarization direction. To improve the capability of target recognition, we proposed the linear polarization sum imaging method which is based on the combination of the different polarization images for increasing the intensity contrast between the target area and the background area. In order to capture the linear polarization sum images of a metal sphere, a metal and a ceramic cup, we designed W-band quasi-optical imaging system which can generate the polarization dependent images by manually changing the linear polarization direction of its radiometer receiver from 0 to π/2 by the step size of π/8. The theoretical and experimental results of the linear polarization sum imaging show that it is capable for achieving good image quality enough to recognize the target.
Quasi-optical imaging systems require low blurring effect and large depth of focus (DOF) to get an acceptable sharpness of the image. To reduce aberration-limited blurring, the aspheric convex plano lenses with an aperture diameter of 350 mm are designed in W-band. We analyzed theoretically and experimentally the millimeter-wave band lens characteristics, such as beam spot size, spatial resolution (SR), and DOF, via f-number. It is first used to verify the DOF through f-number in the system-level test with the developed W-band radiometer imaging system. We have confirmed that the larger f-number of quasi-optical lens leads to a larger DOF but a lower SR.
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