Infrared film thickness measurement: comparison with cold neutron imaging

Abstract

Near InfraRed FILM thickness PROfile (NIR-FILMPRO) is an optical technique for non-intrusive measurement of water film thickness. The technique is based on absorption of NIR light. A passband filter centered at 1612 nm was selected to measure isothermal gravity-driven films flowing on a vertical wall with sand blasted surface. Non-intrusive 2D mapping of the film thickness was acquired at 200 fps, with an image size of 320×256 pixels and a spatial resolution of 0.677 mm. Theoretical developments were brought to consider multiple reflections of light in the liquid film providing a more accurate model to compute the film thickness. Further improvements were made regarding the calibration procedure and the image processing. The measurements were compared against cold neutron imaging, an established technique which provides images of the time averaged thickness. An excellent correspondence between the two methods was found. The root mean square of the deviation between the two techniques taken over a region covered by a wavy film was found to be 2.3% of the measurement with film thicknesses fluctuating between approximately 100 and 500 µm. The spatial comparison with cold neutron imaging complements the previous comparisons and validates the application of the technique.