The LWS consisted of three main components, the focal plane unit (FPU), operating at liquid-helium temperatures, and two warm units: the analogue processing unit (APU), which was driving the various mechanisms and powering the detectors in the FPU, as well as processing the resulting signals, and the digital processing unit (DPU), which commanded the LWS via the APU and interfaces with the spacecraft computer.
The FPU itself consisted of three main subsystems: the optical subsystem, the detector subsystem and the Fabry-Pérot subsystem. The optical subsystem comprised a collimator, a grating, and re-focusing optics which fed the detector subsystem. The Fabry-Pérot subsystem, which was situated in the parallel part of the beam, consisted of a Fabry-Pérot wheel carrying two Fabry-Pérot interferometers. The wheel could be set in any of four positions: in one of these, the beam passed through the subsystem unhampered whilst in another, the beam was completely obscured. In the remaining two positions, one or other Fabry-Pérot was placed in the beam and modulated it spectrally.