Water film thickness imaging based on time-multiplexed near-infrared absorption

We demonstrate the imaging of the thickness of liquid water thin films in the 100-1500 µm range at a constant temperature by monitoring the pixel-by-pixel ratio of absorbance at two near-infrared (NIR) wavelengths near 1400 nm detected with a fast framing InGaAs focal-plane array camera. Experiments were performed in reflection mode with films of pure water and water/ethanol mixtures supported on opaque surfaces using two illumination-detection configurations. One scheme uses specular reflection of incident and reflected linearly polarized diode-laser light at Brewster's angle, which enables detection of signal light that has twice traversed the liquid film with negligible interference from unwanted partial reflections of the incoming beams at the front surface interfaces (air/window and window/water for films constrained by a cover plate or air/water for free-standing films). The second configuration located the detection camera perpendicular above the surface where the detected light was transmitted through the sample and diffusely scattered from the support surface. Imaging measurements of film thickness using both configurations were successfully demonstrated. Time-resolved measurements capture the dynamics of flowing water films or waves generated by droplet impingement.