One of the most important problems encountered by face recognition systems is occlusions. Especially with the COVID-19 pandemic, recognition of masked faces has been attracting interest. There are proposed visible spectrum recognition algorithms, which perform the recognition task from the uncovered part of the face. However, reliability and accuracy of these algorithms does not reach the level of traditional face recognition algorithms, even though they outperform human observers. Furthermore, these approaches do not provide a solution when almost all the face area is covered. In the past, use of millimeter/submillimeter (SMMW/MMW) and terahertz (THz) imaging has been considered as an alternative to visible spectrum systems as an answer to the limitations of visible spectrum systems, such as changing lighting conditions and occlusions. In this study, several performance characteristics of an active THz imaging system operating at 340 GHz are presented for a face recognition approach based on a similarity comparison of THz face images. Here we examine the dynamic range, contrast resolution, spatial resolution, pixel resolution and noise level of the imaging system. Furthermore, analysis results performed on a set of THz images of head areas of 20 individuals are presented. Results indicate that THz imaging is sensitive to facial characteristics through clothing material which can be exploited for a biometric approach based on THz imaging for recognition of concealed faces.
This work consists of the generation of free-space Laguerre-Gaussian (LG) modes at millimeter-wave frequencies by use of a 3D-printed spiral phase plate and measuring the transmission capability of LG beams with respect to conventional Gaussian beams in scattering media such as wood, drywall, and rocky soil in the 1-3mm wavelength range. When a Gaussian beam of light passes through the spiral phase plate, the phase of the light is modified according to the thickness of the etched spiral pattern, resulting in an LG mode. Transmittance of LG beams at both 90GHz and 270GHz is measured with rocky soil, wood, and drywall. The results show that the frequency dependent transmission of the beams is governed by the mode number as well as the anisotropy of the medium. A frequency-dependent improvement in transmission is observed for highly scattering media with respect to mode number. These results support recent studies that show that OAM modes are useful for measurements through anisotropic media. Their demonstration for lower orders, as shown here in the mm-wave frequency range, is exciting for applications such as the detection of concealed objects, wireless communication, and imaging in highly scattering media.
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