The main components of an NDIR sensor are an infrared source, a sample chamber, and an infrared detector. 

The IR light passes through the sample cell and the reference cell and reaches the detector.  

The reference chamber is filled with a gas, usually nitrogen. blood gas analyzer. 

The gas that is measured is called the sample gas

This sample gas passes through the sample cell. A chopper motor is used to split the source IR rays to pass through both the sample and the reference cells.