A Telecentric Offset Reflective Imaging System (TORIS) for Terahertz Imaging and Spectroscopy

Abstract

Terahertz (THz) imaging has emerged as a promising technology in medical diagnostics, thanks to nonionizing radiation and the high sensitivity of THz waves to water content. However, in vivo, THz imaging system designs face challenges, such as slow mechanical scanning, limited field-of-view, and variable incidence angle due to poor telecentricity. To address these limitations, we present the telecentric offset reflective imaging system, a novel dual-mirror scanning design optimized for high-speed, distortion-free imaging. Utilizing a telecentric $f-\theta$ lens and ray-tracing and physical optics simulations, the system achieves uniform resolution across a 50 × 50 mm$^{2}$ field of view. System capability is demonstrated through broadband spectral imaging of a USAF resolution test target across WR-2.2 (325–500 GHz) and WR-1.5 (500–700 GHz) rectangular waveguide frequency bands, achieving consistent beam focus and minimal distortion, with maximum deviation of 2.7$^{\circ }$ from normal incidence and beam waist of 2.1 $\lambda$ at the edge of the field of view. Hydration sensitivity is validated by imaging wet tissue paper, illustrating its sensitivity to temporal changes in water content. Further, in vivo, imaging of human skin after capsaicin patch application reveals localized hydration variations influenced by biochemical responses and adhesive patch removal.