A device allowing direct optical measurement of the displacement was employed to measure the thickness of the liquid-like layer. The data were correlated with calculations of the thickness based on admittance data of the quartz crystal microbalance, measured simultaneously. Agreement between the two independent measurements lends strong support for the validity of the models assumed in our calculations. The approach developed here allows the direct measurement of the thickness of the LLL in the range of a few tens up to few hundreds of nanometers.