Gas measurement sensor including: a measurement chamber including an inlet and outlet for a gaseous sample to be measured; an infrared light source producing substantially collimated infrared light and to direct the infrared light into the measurement chamber; a beam splitter situated in the measurement chamber so as to receive the infrared light and to split the infrared light into a reference beam and a measurement beam such that the measurement beam has a longer pathway through the measurement chamber than the reference beam; a reference infrared detector arranged to receive the reference beam; and a measurement infrared detector arranged to receive the measurement beam. The beam splitter is arranged to reflect a first portion of the infrared light by specular reflection so as to form the reference beam, and to reflect a second portion of the infrared light by specular reflection so as to form the measurement beam.