Zhu, Ting, Shuting Fan, Jian Huang, Bin Ye, Di Zhao, Guangyou Fang, and Xuequan Chen. "Reference-free humidity sensing based on terahertz time-domain spectroscopy." Sensors and Actuators B: Chemical (2026): 139500.
Abstract
Humidity sensing is crucial across various applications, yet conventional electrical sensors rely on physical contact, limiting their accuracy, response speed, and spatial flexibility. Water vapor exhibits dense rotational transitions in the terahertz regime, making terahertz spectroscopy an attractive tool for humidity sensing. However, absorption-based spectroscopic sensing demands a frequently measured zero-concentration reference to extract intrinsic absorption spectra, hindering its practical deployment. In this work, we propose a reference-free humidity sensing method based on terahertz time-domain spectroscopy. The method employs a database-comparison strategy empowered by a two-step calibration procedure, which efficiently adapts the database to variations in optical path length, temperature and spectral response. By comparing any newly measured signal with the database, humidity levels can be directly determined from raw signals without reference and absorption models. The method achieved a root mean square error of relative humidity down to 0.62 % across various measurement conditions and systems. Furthermore, its contact-free nature enables spatially averaged and real-time response, as demonstrated in the respiration monitoring measurement. Overall, the technique outperforms conventional humidity sensing devices over multiple aspects, showing strong potential for applications in medical diagnosis, extreme industrial environments and advanced scientific instrumentation.
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