Direct and indirect measurement of liquid film thickness by common optical methods

Liquid film phenomena widely exist in nature and industrial processes. They have the advantages of low flow velocity, small temperature difference, high heat flux density, and high heat and mass transfer efficiency. For these reasons, they are widely used in various engineering processes. The highly accurate measurement of liquid film thickness, which is an essential parameter for studying the flow characteristics and heat and mass transfer laws of liquid films, is important for the design and optimization of related industrial equipment and processes. This article introduces several commonly used non-intrusive methods for measuring liquid film thickness from the perspective of optics, discussing their measurement principles, advantages, and limitations. The measurement methods are categorized into direct and indirect measurement methods. Direct measurement methods primarily include total internal reflection, laser focus displacement meter, and laser interferometry. Indirect measurement methods include laser-induced fluorescence, Raman scattering, and tunable diode laser absorption spectroscopy.