Thickness measurement of transparent liquid films with Paraxial Self-Reference Interferometry

In this paper, we introduce a non-invasive optical method, named Paraxial Self-Reference Interferometry (PSRI) for thickness measurement of liquid films. The method can be used for thin or thick layers (from μm to mm) of solids or liquids, with a high precision. The method is first applied to solid plates with known thickness and is verified to be accurate. Then we use it for the thickness measurement of liquid films in two experiments. The first experiment is spin coating and the second is dip coating. In both experiments, the results are in agreement with theoretical and experimental results of previous works. In the dip coating experiment, the Landau-Levich-Derjaguin law (LLD) is observed in low capillary numbers, and a deviation from this law due to gravity is seen in higher capillary numbers. The thinning due to the drainage is also observed and is consistent with theoretical predictions.