Absorption of far-infrared radiation, mostly by water vapor, can severely limit its propagation over even short paths.
The above plot is an FTIR measurement of an absorption cell with 10 cm path length. Humidity was not measured but is
relatively high. This cell is located inside the transport system's diagnostic box, and can be remotely inserted ahead of
a detector. By comparing the pattern of lines during a short scan, the operator can measure the FEL's wavelength with good
accuracy. This can also serve as a useful guide in choosing wavelengths, should a long atmospheric path be unavoidable.