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This study proposes a near-infrared (NIR) imaging method for two-dimensional distributions of water vapour (WV) in the open air. A wavelength of 1872 nm, at which the dependence of the absorption coefficient on the WV concentration is the largest in the ν 2 + ν 3 band of water molecules, was employed for imaging. The measurement system was a star imaging system, comprising a laser light source and an NIR camera. A measurement space with 0.08-m diameter and 0.30-m length was ensured to obtain the 2D absorbance images of moist air emitted from a gas nozzle and the breath air exhaled from a human subject with and without a mask. The images clearly showed the distributions of WV that flowed around a pipe and impinged on a plate. The exhaled air was clearly visualised, and the difference in air permeability between the masks was demonstrated. These results indicate the potential of the NIR imaging method for various applications that require WV flow visualisation, WV source identification, and evaporation process observation.

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