Gas measurement detectector configured to monitor a level of a gaseous molecular species within a flow path (18) of a flow of breathable gas that communicates with an airway of a subject is performed by infrared spectroscopy. Rather than using an individual source (20) to generate electromagnetic radiation at reference and measurement wavelengths, a near-infrared electromagnetic radiation source (22) is used to generate reference electromagnetic radiation. The detector comprising: a first source (20) configured to emit mid-infrared electromagnetic radiation a second source (22) configured to emit near-infrared electromagnetic radiation source optics (24) configured to combine mid-infrared and near-infrared electromagnetic radiation emitted by the first and second source into a coaxial beam, and to direct the coaxial beam across the flow path (18) sensor optics (38) configured to receive electromagnetic radiation in the coaxial beam that has traversed the flow path (18), and to divide the received electromagnetic radiation into first radiation that includes mid-infrared electromagnetic radiation and second radiation that includes near-infrared electromagnetic radiation a first radiation sensor (40) configured to receive the first radiation, and to generate output signals conveying information related to a parameter of the mid-infrared electromagnetic radiation in the first radiation a second radiation sensor (42) configured to receive the second radiation, and to generate output signals conveying information related to a parameter of the near-infrared electromagnetic radiation in the second radiation and a processor (36) configured to determine a level of a gaseous molecular species within the flow of breathable gas in the flow path based on the output signals generated by the first radiation sensor (40) and the second radiation sensor (42) such that the output signals generated by the second radiation sensor (42) are implemented to compensate optical loss through th
