Infrared technique for measuring thickness of a flowing soap film

In conducting two-dimensional laminar and turbulence experiments, use of a vertical flowing soap film is often a good choice. However, one of the most frequently encountered and yet highly nontrivial problems is to measure the thickness of the film precisely. We propose a solution to this problem based on the strong absorbtion of infrared light by the water molecules in the film. At $\text{λ≃3\hspace{0.05em}μ}\mathrm{m},$ a thin sheet of water is essentially opaque. The extinction length of 0.9 μm serves as a precise ruler for gauging the film thickness. Although only the time-averaged, single-point measurements are presented, the technique is general and can be used for multipoint measurements to investigate turbulent driven spatiotemporal fluctuations of the film thickness.