*Measurement of the thickness of the thin films relies on the measurement of absolute reflectance. A reference sample with known reflectivity – typically, Si or quartz- is used. This establishes the relation between measured intensity and absolute reflectivity. The procedure is refered to as “taking baseline” or “calibration”. The measurement conditions has to be maintained unchanged for calibration to remain valid. This means, in particular, that the distance from the lens to the sample should be the same as during the calibration. It is especially important for thicknesses < 50 nm