Estimation of the liquid film thickness by the Beer-Lambert-Bouguer law

This paper presents the method of estimating the spatial thickness of a stationary or flowing liquid film by the optical method based on the estimation of the monochromatic light beam attenuation when it propagates in an absorption medium, better known as the Beer–Lambert–Bouguer Law, which is also used in infrared spectroscopy method. The results of the test experiments show that the Beer–Lambert–Bouguer Law is correct if the light intensity is replaced by the value of the brightness component. An estimate of the brightness component is easy to obtain based on the analysis of digital photographs or video recording, namely, by tuples of the value of gray gradation (brightness) calculated using one of the mathematical models for the color representation. For unsteady flows of a liquid film, analyzing the extremum of the autocorrelation function from the displacement of the local area between two adjacent video frames, it is possible to estimate the volume-average value of the flow velocity.