In the case of FEWS, absorption takes place at the interface between the sample and an IR-transparent sensing element, rather than through a very thin sample (as used in standard absorption measurements in the MIR). The beam propagates by a series of total internal reflections at the waveguide/sample interface. The interference between the incident and the reflected waves gives rise to standing waves inside the waveguide, perpendicular to the interface, and to the generation of an evanescent field inside the sample. The evanescent wave decreases exponentially as it propagates farther away into the sample. If the evanescent wave is partially absorbed by the sample at specific wavelengths, the reflectivity is lower than 1. This is called Attenuated Total Reflection (ATR), and it is the basic for the evanescent wave (EW) spectroscopy. The light transmitted through the waveguide is internally reflected several times along its trajectory