Measuring liquid film thickness based on the brightness level of the fluorescence: Methodical overview

Abstract

Thin layers of liquid (liquid films) appear in a variety of industrial applications. The main parameter of a liquid film is its local instantaneous thickness, which can be measured using a great number of methods. One of such methods is based on the phenomenon of Laser-Induced Fluorescence: the fluorescence intensity is integrated along the optical ray and recalculated into film thickness using Lambert-Beer law. An overview of previous applications of this technique is presented here together with a detailed description of this method, including its working principle, factors affecting the fluorescence, calibration procedure, and spatial configurations. The advantages and shortcomings of the technique are discussed. The advantages include high spatial resolution, large area of measurement without coarsening the measurement accuracy, three-dimensional measurements of film thickness, and studying droplets and bubbles simultaneously with the waves. The shortcomings are mainly related to optical distortions appearing locally due to steep slope and high curvature of the interface. These distortions are analyzed in frames of a simple ray-tracing model, showing the limits of applicability of the technique.