Gas concentration measurement apparatus

A gas concentration measurement apparatus 1 comprises a gas cell with a body having gas inlet 3, a gas outlet 4 and a closed void 5 therebetween, a primary light source 20 emitting monochromatic light of a first wavelength, a secondary light source 22 emitting monochromatic light of a second, different wavelength and a primary light sensor 21. The sources 20, 22 may emit square waves in antiphase. The wavelength of the first source 20 may correspond to an absorption peak of the gas to be detected, for example carbon dioxide. The light sources 20, 22 and sensor 21 are connected to the void 5 via optical fibres 12, 13. A reference detector 23 may receive light from the sources 20, 22 that has not passed through the gas in the void 5. Values of measured and reference light from each of the sources 20, 22 may be used to calculate a gas concentration.