A detailed description of the LWS instrument can be found in the LWS observers manual. Here we only give the most important components of the instrument.
The LWS instrument consists of a medium resolution grating spectrograph for
wavelengths between 42 and 197 
m and a high resolution Fabry Perot
spectrograph for wavelengths between 47 and 197 m. The Fabry Perot
uses the grating as an order sorting filter. The detector subsystem
consists of a block of ten detectors, each of which is sensitive in one
wavelength band such that a full LWS spectrum is covered by the ten detectors.
A full spectrum will therefore be divided into ten subspectra (one for each
detector). For the calibration of the data set of five internal illuminators
is included in the instrument to correct for drift effects in the detectors
during an observation.