Pixel-dependent laser-induced fluorescence method for determining thin liquid film thickness distribution

Thin liquid films on free surfaces or inside restricted spaces are widely encountered and can have significant effects in heat transfer, coating, biofilm growth, and lubrication. The thickness and distribution of these thin liquid films are important determinants of performance, but cannot easily be measured accurately. This paper presents a methodological study on determining the thickness of thin liquid films by means of laser-induced fluorescence. A model of fluorescence imaging and measurement is established based on the fluorescence energy transmission. A novel pixel-dependent laser-induced fluorescence method is then proposed and compared with commonly used methods using experimental test results. The accuracy and precision of the measurements are discussed in detail. It is found that the measurement performance is greatly influenced by the nonlinearity, heterogeneity, and random noise of the optical system and imaging technique. The proposed pixel-dependent laser-induced fluorescence method effectively eliminates or ameliorates the effects of these factors and enhances the measuring accuracy.