A filter-based Raman spectrometer for non-invasive imaging of atmospheric water vapor

A new instrument was designed and developed to map the spatial distribution of water vapor concentration in the atmosphere. The high spatial resolution, sensitivity, and accuracy of the instrument enable new studies of the role of turbulence on clouds and aerosols in small-scale laboratory environments. The instrument exploits Raman scattering in a multi-pass laser configuration by using a set of narrow bandpass filters and a pair of charge coupled device imaging cameras in the 90° scattering geometry. The absolute concentration of water vapor was inferred from measured ratios of H₂O and N₂ vibrational Raman transitions. We have measured the number densities of water molecules in the atmosphere as low as 3.5 × 10¹⁷ cm^-3, with an accuracy better than 20% and as high as 7.0 × 10¹⁷ cm^-3 during minutes long observations. These measurements were taken within an imaging region 6 cm in diameter, with a per-pixel resolution 2.60 mm wide by 0.16 mm tall and 1 mm deep.