An emerging application of terahertz radiation is industrial process control. In paper production lines, for instance, a variety of parameters has to be monitored, including temperature, humidity and paper area density.

Terahertz-based measurements might provide the long sought-after alternative to commonly used radioactive emitters: Terahertz radiation is hardly attenuated by paper, but strongly absorbed by water [1]. More precisely, both the transmittance and the terahertz phase vary with the humidity level of the sample. The figure shows the drying process of a sheet of tissue paper, monitored at a frequency of 200 GHz. After approx. 20 minutes, the sample was sprayed with a detergent solution, which is seen as a sudden step in both transmission and phase data. During the following hour, the two parameters converge towards their initial values as the sample dries [2].