The light monitor is used to determine the effective mid-moment of the exposure. This procedure makes the correction of the variation of the radial velocity by the motion of the Earth much more accurate.
The light monitor consists of a beam splitter, photo-multiplier and a pulse counter. When the instrument is inserted to the beam, about 1% of the beam is extracted by a beam splitter and measured by a photo-multiplier. The photo-multiplier is driven by a 5V power supply which sends the output measured to the pulse counter.