**3. Results**

The prototype instrument elaborates acquired signals for obtaining the more accurate thickness measurement. Figure 12 shows an example of raw signal acquired while measuring a transparent plastic film, with nominal thickness of 50 μm. In the figure, the two couple of peaks are evident, corresponding to the ascendant and descendant phases of the triangular modulation. Time distance between two peaks is proportional to the film thickness.

With regards to the amplitude variability of every peak, we have to note that the peak's amplitude depends on the angular alignment of the sensor, on the parallelism of the two surfaces, and also on the absolute distance (the relative position with respect to the focus). For these reasons, in every measurement, we can see different amplitudes, which are also relative between the peaks corresponding to the two surfaces. To pass from time measurements to thickness measurement, an instrument calibration is required, as described in the later paragraph.

**Figure 12.** Raw signal acquired for a transparent film; nominal thickness 50 μm.
