Calibration of the water film thickness. (a) The original (air) signal from the autocorrelation system is plotted in the black line. With the water film inserted in one of the two laser beams of the autocorrelation system, the signal peak will move linearly with the increase of film thickness. (b) The linear dependence of the film thickness on the flow rate. The thickness of the water film can be continuously varied from 50 μm to 350 μm.