The CO sample concentration is determined by measuring how much infrared light the sample gas absorbs as it flows through a multi-cell correlation wheel filled on one side with a reference CO cell (reference beam) and on the other side with an empty cell (the measure beam). As the wheel turns round, the light beam passes alternatively through the CO cell and the empty cell and then through an interference optical filter before reaching the optical detector. If the sample contains CO, the reference beam will not be attenuated by it because it was already attenuated by the CO of the reference cell. The measure beam however will be attenuated by the CO in the sample.